I'll try though to sift through and break down the mechanical and gameplay aspects of it though. For some reason I got a wild hair to do this for the games that I'm currently into or reading. It bears mentioning that I did a post that hit on alot of the mechanical/game aspects of TSOY here.
The Shadow of Yesterday [wiki] is a game that's actually available for free, thanks to its author, Clinton Nixon and the Creative Commons License. You can grab the text of the main book here, and the setting here.
Firstly, damn, I cant say enough good things about this game. I'm totally hooked.
TSOY is a rather rules-lite fantasy RPG. It uses FUDGE dice.
Character creation is pretty quick and simple. You'll divide 11 points among your three Pools, Reason, Instinct and Vigor. I'll fill you in on what these do shortly. Abilities (skills basically) are all ranked Unskilled (rank 0), Competent (rank 1), Adept (rank 2), Master (rank 3) or Grand Master (rank 4). Everyone takes Endure (based off of the Vigor Pool), React (based off of the Instinct Pool), and Resist (based on the Reason Pool). They take one at Adept, one at Competent and one at Unskilled. Then you choose four more abilities, and there are excellent examples provided in the book, enough that you dont have to create your own, though you're encouraged to do so, if you find yourself in need of one that's not printed. Of the four additional abilities that you take, you get one at Adept and the other three at Competent. You also get one Secret and one Key. Secrets are the equivalent to Edges or Feats. They let you hit harder, jump higher, be more convincing, etc etc. Keys are part of where the game really shines. A Key is something like the Key of Bloodlust. With this key, every time your character defeats someone in battle, you gain 1 xp; or 3 xp for defeating someone who is equal to more powerful than your character. In short, Keys are how you define the game for yourself. If you want to play a character who is all about the brawling and fighting and blood, you take Keys like Bloodlust, Masochist and maybe Conscience. Thus you're getting XP whether you're beating people down or getting beaten down yourself. Or if you're a more thiefly type, you can take a Key that gives you XP for stealing stuff and getting gold. Its really really the most excellent thing. There are a number of good keys in the book, and you're encouraged to make up your own. Someone familiar with the game could have a new character ready to go in like three minutes probably, and a first timer with some guidance could have one done in five or ten mintues, allowing for time for them to read through the abilities, secrets and keys.
Since they're so cool, here are some more of the keys written up in the book:
*Key of the Coward
Your character avoids combat like the plague. Gain 1 XP every time your character avoids a potentially dangerous situation. Gain 3 XP every time your character stops a combat using other means besides violence. Buyoff: Leap into combat with no hesitation.
*Key of the Impostor
Sometimes your entire life is a lie. You gain 1 XP whenever you pass yourself off as someone/something you're not. You gain 2 XP whenever you convince others in spite of serious skepticism. You gain 5 XP whenever your story survives a deliberate, focused, "Hey everybody, look!" attempt to reveal your identity. Buyoff: Confess your imposture to those duped.
*Key of Renown
"You must be the worst assassin I've ever heard of." "But you have heard of me." You gain 1 XP whenever you see to it that your name and deeds are known, by bragging about them or making sure there are witnesses. You gain 2 XP whenever you put yourself at risk to do something unnecessary or foolish that will add to your reputation. You gain 5 XP whenever you risk your life to take credit for your actions (bragging that you were the one who killed the Duke's son, for example.). Buyoff: Give someone else credit for an action that would increase your renown.
Gameplay is pretty simple, but has a twist, which I'll explain. Every dice roll is going to be a conflict of some type. The system is setup for Conflict Resolution, rather than Task Resolution. The clearest example of this is combat, in which instead of declaring that you're stabbing the nasty orc, then rolling to hit and doing some damage, then repeating the process until it falls down, you create stakes and roll the conflict. It might work like this. "The nasty orc is going to stab your eyes out! Conflict time. Lets set the stakes as follows- if you win, your quick work with your shortsword leaves the orc in a bloody messy pile in the dark alley. If you lose though, the orc is dead and in a messy pile at your feet, right as his four friends come around the corner. Roll." Or something similar. There are innumerable variations, all depending on your groups creativity and scope of play. Basically though, while failure at a conflict can mean failure at the task, it more often, or perhaps should result in a complication. If you're picking a lock in the middle of the night and you roll poorly, it doesnt necessarily mean that you just failed this time and should try again. Heck maybe the door clicks right open, but the owner of the propery you are breaking into and his two mean nephews are just inside and happen to be sharpening their new swords. Complication :)
As far as the actual dice rolling, its Fudge dice, so they're six sided dice, but instead of the sides being numbered 1 through 6, you have two faces with a "+" on them, two with a "-" on them, and two blank sides. By default in a conflict you roll three Fudge dice. I'm sure you can do the math, but with three of these dice you may get a +++ or you may get a ---, or anything in between. You take this result (a plus, a plus and a blank is 2; a plus a minus and a plus is 1, three blanks are 0) and add it to your rank in the ability. All you need to succeed is a 1, unless its a contested roll. So if you're Competent in the ability, you only need three blanks to succeed, since Competent is (1).
Now the twist. Bringing Down the Pain, or BDtP. Any player can elect to Bring Down the Pain after a resisted ability check. Usually its done after a failed check. What it does in regard to the game is allow the player to "do over". But more than that, it allows them to focus the invisible camera of the game onto them and what they're doing. It breaks from the Conflict Resolution system of the game, and becomes more task based and granular. It uses what's called the Harm Tracker to determine the outcome, though of course either opponent can give, and allow the others intent to take place. I'll talk more about the Harm Tracker in a sec. Lets say you're a sneaky thiefly type, trying to sneak your way into the Baron's castle. Well there's a guard walking a regular patrol on the parapet that you just scaled, and you need to get past him. He's keeping an eye out for people just like you, so we have a conflict (unless you Said Yes, as in "Say Yes or Roll Dice" another separate but fun topic). We setup stakes - you're going to use your Stealthy ability, which you're a Master in (that's 3), and I'm going to use the guard's React ability, in which he is a Master as well. Stakes are that if you succeed, you've grasped the lip of the parapet, waiting until the guards patrol carries him past your position, then once clear you launch yourself up over the battlement and land on the balls of your feet, sprinting silently across and leaping over the opposite battlement to catch hold of the edge so that you can climb down the inside of the castle's wall, if you fail I decide that you leapt onto the parapet right after the guard passed, but that a flagstone shifts under your foot, alerting not only the guard that just passed, but also the Captain of the Guard and his two cronies who you didnt notice until just now.. We roll and I get a plus, plus, blank, so a 5, while you get a plus, minus, blank so a 4. Well you could certainly stay with the results of the failure. It might be alot of fun to figure out how to get out of that predicament. But instead you can ignore the result and Bring Down the Pain. We rewind, you're still clinging to the outside wall of the castle. There's a guard walking patrol on the top. We break it down and set intents, yours is to get across the wall unseen, mine is to discover the thief hiding about in the shadows. You decide that you're going to use your Deceit ability to provide a distraction, throwing a small rock down the parapet to draw his attention. In turn, I'm going to use his React ability to keep a wary eye out on the shadows, looking for anyone trying to infiltrate the castle. You notice these are Parallel actions - they dont actual counter each other, if that were the case we would call them Perpendicular. So you roll your Deceit and I roll my React, the guard ends up with a 2 and you end up with a 3. Since these are Parallel, we both deal harm to each other, there's no defending or subtracting anything in this case, so I take Harm at level 3 and you take Harm at level 2. Next round we'll both have a penalty dice. At any point either of us could give, allowing the other's intent to take place. But we're going to keep going, so you decide that you're going to use your Athletics now to move hand over hand down the wall, further away from the guard in order to get away from his gaze, while the guard continues to keep a close eye out, made nervous by the clattering rock. We roll our abilities and perhaps in this case, the thief might use his successes to subtract from the number of successes that the guard got, thereby reducing my harm - since the thief is moving away from the guard. We continue back and forth like this, using any abilities that we can bring into the action here and going back and forth with dice rolls until one of us gives, or until we're taken past 6 on the Harm Tracker. And that's another of the really really beautiful things about the system. The harm tracker is not just another clever take on Hit Points. Its not just how many hits from a sword you can take. It is used whether you're having a spear duel against barbarians from the north, trying to talk the king into considering your plan, or attempting to sneak past guards on top of walls.
So I went kinda long there. I sorta get a wild glint in my eye and begin to gesticulate and froth at the mouth when it comes to TSoY. You'll have to pardon me. But that's really it. That's the system. Players hit their keys and grab XP (I use pennies and nickles, penny = 1 xp, nickel = 5 xp, easy enough). When they hit them, they reach in and take XP. You can use those Pools I talked about earlier to add a bonus dice to your pool for a conflict. Oh, and there are gift dice. At the beginning of every session, you get a number of dice equal to the number of players at the table that you can give out once per conflict to other players when they're doing something especially cool or groovy. They roll that as a bonus dice, increasing their chances for success.
I cant say enough good things about this system.
[edit - fixed a couple errors]
[edit - more on keys!]