e6 is a modified ruleset for D&D which makes level 6 the "epic" level - at level 6, they stop levelling, and instead gain feats via XP. This means heroes reach a point where they are powerful in the face of worldly dangers, and yet never reach ridiculous echelons of power.
From the Dungeons and Dragons wikia's introduction to e6:
What is E6?
Ryan Dancey suggested that d20 has four distinct quartiles of play:
- Levels 1-5: Gritty fantasy
- Levels 6-10: Heroic fantasy
- Levels 11-15: Wuxia
- Levels 16-20: Superheroes
There’s been some great discussion about how to define those quartiles, and how each different quartile suited some groups better than others.
E6 is a game about d20’s first two quartiles, and focuses on continuously delivering exciting heroic fantasy, even in a very long campaign. Like d20, E6 can be adapted to a wide variety of settings, from high-magic action-adventure where magic takes the place of technology to low-magic worlds where sorcerers are spoken of in frightened whispers and dragons are the stuff of legend. E6 keeps all the benefits and familiarity of low-level d20 games: Fast-paced combat, quick prep, and an incredible wealth of third-party material that can be used with the game. E6 has been playtested extensively, and its rules that can be explained to veteran d20 players in under a minute.
More information on e6 is freely available on the Dungeons and Dragons wikia.