All Music Guide
http://www.allmusic.com/

Blues Archive (University of Mississippi)
http://www.olemiss.edu/depts/general_library/files/music/bluesarc.html

Chicago Jazz Archive
http://www.lib.uchicago.edu/e/su/cja/

Encyclopedia of Music in Canada
http://www.nlc-bnc.ca/music/17/index-e.html

IndieBlast - MusicCool (Canada)
http://www.indieblast.com/musicscool/index.html

Internet Underground Music Archive http://www.iuma.com

Mudcat Cafe
(Blues and Folk articles and lots more!)
http://www.mudcat.org/

Music Software - Shareware Music Machine http://www.hitsquad.com/smm/

Napster (what else can I say!) http://www.napster.com

National Ragtime and Jazz Archive (US)
http://www.library.siue.edu/jazz/

Rock and Roll Hall of Fame and Museum
http://www.rockhall.com/museum/

Roughstock (Online Country Music) http://www.roughstock.com/roughstock

Sheet Music Recommendations from Shareware Music Machine
http://www.hitsquad.com/smm/sheetmusic/?nl52

Sheet Music Plus
http://www.sheetmusicplus.com