The engine for 5 Second Game is built on a Rock-Scissors-Paper concept. There are three stats, corresponding to that, and everything modifies that: character classes, skills, combat, etc. It is meant to be a fast paced, easy to grasp, infinitely extensible game.
The game is designed to be fit into an embeddable widget, so that users may post the game in a blog or other page, allowing others to quickly and easily play the game. A widget may highlight that user's character, challenging all takers.
The game lends itself well to guilds and the like as well, which are also built into the system. Characters may belong to one or more guilds, which govern themselves as desired by their players. The system honors the guilds' wishes, so that characters belonging to a specific user-formed guild might gain certain bonuses (or suffer penalties) according to the guild charters.
Additionally, servers may choose to allow characters to migrate between certain other servers. The character may be "translated" during migration, so that world/genre integrity is maintained (if desired). However, the simplicity of the system lends itself easily to that, if an administrator doesn't mind a gun-toting sheriff to arrive at the scene of a medieval tournament, or conversely if the player of a high elf wizard doesn't mind being translated to a nanobotic engineer on a space station.
You can play the game at http://5secondgame.com to see it in action (once it's built...)