Boss Sounds
This is a really good site with some history info on the differences between ska, rocksteady, and reggae. With sound clips and more.

Ska2Soul
This site has some very good history on ska with listings of rare albums from the first wave, bios on some influential artists, and lots of other stuff. Even flyers from shows in the 60's

This site has information on early ska and reggae history, with some cool histories of some of the early record labels.