Napster
Shareaza
Soulseek Records

General Resources

All Music Guide
Biograph Records
Bubl Link Music Page
CD Hotlist - a music recommendation service for librarians, from librarians.
Cheap-cds.com--no frills, AND they seem to have everything at decent prices. Also you can listen to MP3 song samples.
Children's Music Network
Digital Librarian: Music
Dusty Groove - an outside-the-mainstream retailer in Chicago
eDionysus Music
Hypermusic
Instrument Encyclopedia is "sampler of some of the world's most interesting musical instruments," not a comprehensive collection.
Internet Underground Music Archive
Musical History (and more) from the Smithsonian Institution.
People's Music Network
Shanachie Entertainment 
Worldwide Internet Music Resources

Acoustic/Folk/Traditional

Acoustic Disc
iBluegrass.com
Bluegrasscountry.org 
Bluegrass Lyrics - lyrics for bluegrass and traditional country music.
Bluegrass World
Dirty Linen - folk and world music
Folk and Acoustic Music Exchange
FolkLib Index - a Library of Folk Music Links
FolkMusic.org
Folkstreams.net
Folkthings - ethnic, traditional, & folk dance, music, & arts
Hobgoblin Music  Information about folk, traditional and celtic music and musical instruments
House of Musical Traditions  Acoustic instruments, accessories, & books from around the world. 
Musical Traditions
Old Time Herald - "celebrates the love of old-time music -- grassroots or home-grown music and dance."
Rounder Records
Sugar Hill Records
Smithsonian Folkways

Blues

Alabama Blues Project
Alligator Records
Arhoolie Records
Arkansas Delta Family Gospel Festival, Sat., May 24, 2003, in Helena, Arkansas. FREE!!! 
Blue Highway
Blues Festivals - links by month to festivals around the country.
BluesNet
Blues Planet Records
Fat Possum
Freedom Creek Festival, June 14, 2003 in Old Memphis, AL, near Aliceville, AL. Willie King's annual fest.
House of the Blues
Howlin Wolf Memorial Blues Festival
King Biscuit Blues Festival
Magic City Blues Society
MCA/Chess
Mississippi Delta Blues and Heritage Festival
Mudcat Cafe  A magazine dedicated to blues and folk music.
Shannon Street Blues and Heritage Festival, May 30 & 31, 2003, in Jackson, Tennessee. 
Summerfest BBQ and Blues May 23-24, 2003, in Hollandale, MS, home of blues legend Sam Chatmon. FREE!
Sunflower River Blues and Gospel Festival
W.C. Handy Festival
Yazoo Blues

Classical

AllClassical.com
Classical.net
Classical Music Navigator
Classical Music Pages  Searchable by Epoch, Musical Form, Composer's Name, and ultimately someday by instrument. 
Essentials of Music  Good site for students in that it allows exploration of classical music by era, or by composer. And there is a glossary.

Jazz

AllAboutJazz
All Music Guide: Jazz  Divided nicely by the main types.
Jazz- the companion site to the film by Ken Burns.
Jazz Hall of Fame
Jazziz
Jazz Kids, from PBS
Jazz Online
Jazz Roots
NPR Jazz
A Passion for Jazz includes a good timeline in the History.
Red Hot Jazz Archive
WNUR FM Jazz Web, from Chicago

Roots, Reggae and World Music

cdRoots
FRoots - "the world's leading roots, world music and folk monthly."
International Music Archive - "an educational resource providing extensive information about the music of our planet."
Reggae Train 
Reggae Reviews 
Roots Archives -  a comprehensive and searchable database of Jamaican Roots Reggae Albums from 1970 to 1985. 
Roots and Rhythm
RootsWorld
World Music Central - in the sense of "local music, not from here."