GURPS: Generic Universal Role Playing System
While the sourcebook are excellently detailed, GURPS itself tends to lends itself to a somewhat cinematic, swashbuckler style of play, much like that found in an action movie. It's aged quite a bit better than D&D or White Wolf has, but the style of play it fits doesn't work for most gamers.
FUDGE: Freeform Universal Do-it-yourself Gaming Engine
With no numeric stats and nothing actually set as far as stats or anything but the base mechanics goes, FUDGE is a fantastic engine for making any genre of game you could possibly want. While it takes a measure more of work for the GM than, say, D20, Rifts, or even Cyberpunk 2020, it has incredible versatility.