Sponsored by the University of Pittsburgh’s Center for American Music, this is an excellent site. It provides an intelligent biography of Stephen Foster, a catalog of his works, and access to digitized source material.
This site offers an engaging history of the minstrel show complete with timeline, images, and audio files. Hosted by the Old Time Radio Catalog, the site pursues the minstrel tradition through some of its 20th-century successors, like Amos and Andy.
The California state library has posted an excellent site on the Gold Rush. It provides images and documents useful to the exploration of everything from the discovery of gold to the life of a miner.