FATE is a flexible,tailor-ableRPG engine, with official licenses for Harry Dresden, Diaspora and Starblazer Adventures and Spirit of the Century.
FATE makes a number of interesting design choices; some of which I like and others I don't agree with.
First off: the dice. FATE uses 'FATE dice' (a.k.a. FUDGE dice) which are D6 with two '+' sides, two '-' sides and two blank sides. You typically roll four at a time, thus getting a range of -4 to +4 with an average of 0. Onecoulduse regular D6 and just note that 5&6 are a +, 3&4 and neutral and 1&2 are -.[The only other version of rolling a positive/negative range I have seen is rolling two D6 or two D10 and subtract one from the other: resulting in a range of +-5 or +-9.]My experience with FATE dice, however, is that I hardly ever roll a positive result. Sometimes I have terribledice luck.
FATE sorta balances this with Aspects, which can come from characters, locations and situations. Tapping an Aspect gives a flat bonus to the roll and is a narrative opportunity for the player to explain how the Aspect is a boon to their action. Players can even add new Aspects to themselves, opponents and locations by attempting a Maneuver action; success means they get to place a new Aspect. You can alsocompelother people's Aspects to force a penalty to their rolls, or straight dictate their actions.
Characters in FATE don't have fixed sets of Attributes, stats or ability scores. They are a collection of Skills, rated from Terrible (at -2) up to Legendary (+8). If you character doesn't have it written down, you're considered to have a "Mediocre" (+0) rating in that Skill. The difficulty numbers for attempting actions are rated on the same "Ladder". A "Great" Difficulty task requires +4 or better total. This, coupled with the dice mechanic, provides both a rational and a quantitative way of measuring ability, challenge and the effort a character makes.
Characters also have Stunts. These are rule-modifying or outright rule-breaking 'feats' the character possess. These are highly setting specific: In Harry Dresden the can represent magical powers; in Diaspora they could mean your character is trained on military-grade gear; In StarBlazer Adventures they represent alien abilities and other pulp sci-fi abilities, and so on.
Conflicts in FATE are fairly open structured. Maps are drawn as needed but scale and accuracy aren't necessary; they are for purely relative values/locations. One can even map out social conflicts, which I like. Characters (and other combatants) have Stress tracks to mark 'damage.' Once your stress track is full, you're out of the conflict. Players can opt to take Complications at varying levels to reduce incoming damage. Complications are essentially negative Aspects that opponents can tap in future tests and conflicts. 'Damage' is simply the difference between an attack roll and a defense roll.
FATE also has a dramatic economy measured in Fate points. These tokens of story flow allow players to boost their rolls (or reroll them), as well as tap aspects, complications and create new ones. Compelling someone else's action because of their Aspects costs you a Fate point, which you give to the compelled player if they accept. Fate points are also introduced by the GM for good and dramatic roleplaying.
Character advancement is a bit of an odd duck, but it makes sense in that FATE is a story-focused game, and not a exercise in counting experience points and gold pieces. It also differs from game to game (Diaspora having the lest detailed character advancement system), but in essence: the GM determines when the characters have reached milestones in their story arcs, and at these milestones players may make incremental changes to their characters' Skills. Aspects are the most easily changed; they can be tweaked before a game session even begins as they are highly subjective. Stunts can be swapped out as can your Skill ratings for minor milestones. For major milestones you can add new Stunts or Skills.
Overall, I like FATE in concept. I'm not thrilled with my experience with the dice mechanics, but the conflict system seems robust, flexible andscale-able. Complications and Aspects add a wonderful freeform and subjective element when most games get too detailed and deterministic. I like how character growth is also tied to story, and dissuades players from trolling for every XP point they can get. FATE is another one of those games that I have yet to actually play, but I am looking for an opportunity to give it a go.