if you're willing to give hip hop/jazz a try, get Guru's Jazzmatazz Volumes 1-3. you won't be disappointed. mellow and deep...
also if you want mellow, why don't you try some 70's soul, like the obvious Isaac Hayes, Barry White etc.
that's the stuff i listen to!
EDIT: also try Jurassic 5. despite they had only one hit that came out on radio, they actuallly have some really good stuff....
EDIT 2: for more modern mellow downtempo music,my favorite band is Zero 7. also check out Thievery Corporation, Air, Mo Horizons, DJ Shadow, Portishead, Kid Loco