yea I use gendou to dl opening and endings, but for ost I use nipponsei or box.

Rock hmm.. fma has a few rock opening, tobira no mukou he (yellow generation), rewrite (asian kung fu generation), gundam seed and seed destiny have heaps of those, naruto has some (haruka kanata by asian kung fu generation), shakugan no shana's first opening hishoku no sora by kawada mami is damn good (not sure if it's really rock tho), and as mentioned suzumiya haruhi's op, ed and the concert songs (god knows) are pretty good, especially god knows, tales of abyss's karma (altho not anime, the japanese opening), musouka by suara (utawarerumono), hemisphere by sakamoto maaya (rahxephon), a few rock ones that I can remember for now.