True20

True20 is a role-playing game developed and published by Green Ronin Publishing. It was originally used in Green Ronin's Blue Rose. Blue Rose won the Gen Con ENWorld Roleplaying Silver Medal for Best Rules in 2005. True20 is derived from Wizards of the Coast's d20 system. The main differences are:

There are only three character classes (warrior, adept, and expert).
Only a single 20-sided die is used for every roll.
Ability scores are given as modifiers (+1, +2, etc.) rather than from 3-18
There are no cross-class skills; any class can have any skill
Feats are available every level
Magic spells are done as feats, and do not have levels, but are more akin to 2nd Edition AD&D Psionics
Instead of Hit Points, characters simply go through damage conditions
Instead of experience points, characters automatically increase in level after 1-2 adventures, thereby encouraging more roleplaying instead of "hack & slash"

The True20 Adventure Roleplaying book includes four story settings:

Caliphate Nights, adventuring in a fantastic version of the golden age of Islam, circa 800 AD. A full-color stand-alone hardcover was released for the setting at Origins 2006.
Lux Aeternum, swashbuckling space opera
Mecha vs. Kaiju, giant robots protecting Japan from giant monsters
Borrowed Time, kung fu and gunplay against a backdrop of time-controlling conspiracies

The Worlds of Adventure included 5 more settings