Fanmixes are the 'mixed tape' of fandom involving compilations of music that may be in the style of the classic rock of the show, or mixes compiled as a character study or as a meta on a particular episode or story arc.
For a description of the art of making a fanmix, read memphis86's {[http://community.livejournal.com/spnroundtable/156799.html Workshop at spnroundtable} ] - Fic Soundtracks: Unite Rhythm With Words.
Fanmixes are often posted jsut with fics, but there are also some Fanmix comms: