Depends what you mean by simpler:
Originally Posted by Mulsiphix
GURPS is simpler in the sense of having consistent mechanics and making miniatures combat (and many other rules) entirely optional. It's emphatically not simpler in terms of character generation; creating a character using all the advantages/disadvantages/skills/etc. from the Basic Set is like drinking from a firehose. I'd start with the free "GURPS Lite" rules, and then introduce parts of the Basic Set as needed ... that would get you used to the system, and if you don't like what's in the free rules, you can save yourself $75 for the full Basic Set (never mind genre books).
True20 is a simplified version of d20, with only three basic "roles" (classes) and a few other innovations. It's not a bad system, and it might give you a comparison with "orthodox" D&D.
"d20" is, itself, the Open Gaming License rules of D&D 3.5, plus the Open Gaming License rules of "d20 Modern". The raw System Reference Documents (SRDs) themselves are more for game designers than players; I'd look into published variants like Spycraft (itself notoriously over-detailed), Star Wars Saga Edition, or Etherscope.
Another possibility is to try 1st and 2nd ed. D&D. You can find some materials cheap at used bookstores, and others available as PDFs on RPGNow. Or, you can try "old-school" revivals like "Castles and Crusades".
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)