An excellent site from the Michigan State University ASL Lab. This site uses animiation clips to demonstrate the signs and includes an alphabetical index..
Some quick facts about ASL from the National Institute on Deafness and Other Communication Disorders (NIDCD) website. The NIDCD is part of the National Institutes of Health.
This animated site includes an alphabetical index, selected word categories and animated short stories. There is also a section on language and culture plus a brief introduction to International Sign Language.