GURPS is one of the best RPGs ever written, and the fourth edition is the best edition of GURPS ever written. It's a 3d6 roll under, point based system. The earlier editions had greater strength toward fairly realistic, action-oriented games, but over time, GURPS has covered pretty much everything at this point, including Discworld, real life Cops, and supers.
(....lots of truth.......)
EDIT: and how would you compare it to D&D?
It's faster and more intuitive, and about equally deadly at low to middle power levels. It has about the same learning curve. It has less special cases. The main GURPS Fantasy setting is something of a different beast, very high medievally flavored, very much SCA meets mythical Shakespeare. It has elves, dwarves, halflings, orcs, lizard men, ogres, minotaurs, goblins, hobgoblins, kobolds, etc.
There is also a line of products called GURPS Dungeon that helps you optimize the GURPS experience for traditional dungeon crawls.
GURPS can come very close to being D&D, depending on how you adjust the dials. It can also be something quite different.