Pretty much anything I find useful.
I probably take the most from Grehawk, technically speaking, given the number of spell and magic items in the core rules are named after people from that setting.
I steal a lot from the Realms too. Lots of good spells, monsters, and with 3e feats and cleric domains I can easily rip off for my own game.
I pretty much take any kind of cruncy bits that look good, and shove it in my game. Stuff like spells, monsters magic items, feats and cleric domains are high on the list, because they take the least amount of adjustment to plop into a campaign, and you only need to put it in a little bit here and there, like a spell here, a monster there, and so on. I don't care what setting it is.
Prestige classes get ripped less. A lot of PrCs are campaign specific, so naturally, they get ripped less.
Now flavor, that comes from just about anything. Books, televison, movies, video game, other D&D settings, and my own imagination/dreams.