i've always liked how a nice steady hip-hop beat flowed with some footage. i know there are hundereds upon hundreds of beats on limewire and such that noone really owns rights to, not to mention they put them out there for people to freestyle on, and to use for songs and such.

im pretty sure as long as you credit the artist on the ones not for sale, its perfectly legal to use pretty much any music you want.

the ones that are for sale, however, im sure if you just emailed, myspace messaged, or even called the band or the bands manager im sure they would have no problem with allowing you to use their work.