d20 systems
I've played D&D since first edition, Star Frontiers, Cthulhu, Gamma World (the old one) Runequest, Palladium, and good ol Heroquest (which to me looks like an early forerunner to 3.5 miniatures and battlemaps). Of all the systems i like d20 the best, not because others are bad, but the rules, for our group anyway, took such a LONG time to master that we don't really want to bother doing it again.
That said, i love being able to pick up another d20 system and use it without learning a whole new ruleset. Other posters mentioned the d20 Call of Cthulhu, and here is what i did; when the book came out in 2002 (?) i bought it and the older version and some sourcebooks from Amazon. So, we used the cleaner d20 rules, but spliced with the flavor and tension of the old game. Yeah, the new Cthulhu does make it more action intensive, but the monsters are still so powerful that investigators will still get eaten unless the Keeper gives them "D&D" special abilities (which my playes begged for; they didn't want to be Elder God fodder.)
I've heard that Ars Magica has a great magic system (i like Midnight's spell points myself) and Unearthed Arcana next month from WoTC might have a good spell system. Darwin's World gets great reviews, but like others have mentioned, so long as you're having fun i suppose it doesn't matter whether you branch out or not.