You would have thought that Muxtape (previously mentioned) would have included a "search" functionality, but I suppose the lack of this feature gave the site a certain charm or "authenticity" that reflected the linear nature of the original cassette mixtapes. On the other hand it's the 21st century and it's a real pain in the aural arse to not be able to directly find a particular artist or song.
Someone addressed this issue by creating Muxfind which allows you to search Muxtape for specific artists (no songs). Yay!
And I'm only the messenger here folks: But someone else created a Greasemonkey script that allows you to download, theoretically, as mp3s the songs that others have uploaded to Muxtape.