Sports Announcers Past to Present - Great site with in-depth profiles and sound clips from famous announcers of all sports.
Classic Radio - Good site with very basic information on hundreds of baseball broadcasters.
Call of the Game - A very good site devoted to minor league baseball broadcasters.
Sports Broadcast Pages - Another very good site with links to just about every online professional sports broadcast available.
Sportsjuice.com - A good site with links to many live minor league and other baseball broadcasts.
Major League Baseball Media Guide - A list of broadcast stations and announcers for every major league team (not quite up-to-date).