Rock and Roll Museum and Hall of Fame
http://www.rockhall.com/
An excellent resource with timelines, biographies and stylistic analysis. Your first stop for information.

The History of Rock and Roll
http://www.history-of-rock.com/
This site explores the roots of rock emphasizing the progression of musical styles.

The Rock and Roll Library
http://www.rocklibrary.com/
Their efforts focus on preserving and presenting the historical and cultural significance of popular music.

Rolling Stone Magazine
http://www.rollingstone.com
Rent Almost Famous and see the power that is Rolling Stone magazine.

Pandora
http://www.pandora.com
Tell Pandora what song you like, and it will offer suggestions based on the genes of that song to another song with similar genes. Fascinating and fun.

Last FM
http://www.last.fm
This site recommends music but based on social networks. Enter a song you like, and Last.fm will let you know what other people who like the same song also like.

The University of Lethbridge
www.uleth.ca