Signing Savvy: Free to use, and offers both free and paid memberships. This is a great dictionary that lets you type your entry in a search box rather than browse word lists. Signs are shown clearly and offer the unique feature of written descriptions, which are very helpful. A few more features and unlimited daily searches are available if you sign up and/or pay. Paid memberships are very expensive, however. Also annoying because I seem to have to log in every single time; I'd love it if the site used cookies and could remember me from one visit to the next to save me having to log on to see sign descriptions.
ASL Browser at MSU: This used to be our go-to ASL dictionary online, but lately, we prefer Signing Savvy simply because you can search directly rather than peering through word lists. Offers bare-bones white screens and blocky QuickTime animations that are a bit slow to load, but sometimes offers some signs the other dictionaries don't have.
(a bit out of date now)
ASL Pro: Free website offering an excellent and surprisingly comprehensive dictionary as well as practice quizzes (everyday or religious signs or fingerspelling quizzes) of several length and difficulty levels. Special section for religious signs, including a surprisingly good number of Jewish ones.
Dr. Bill Vicars' Fingerspelling Practice Site: Just found this one now, and boy, does my fingerspelling need practice. Offers other tools as well, which I haven't checked out yet. Fingerspelling quiz can be adjusted for speed and word length - luckily for me!
LifePrint Online ASL Course: Another one I just found that I'm planning to use especially for its vocabulary reinforcement and story-receiving practice, as well as quizzes. Most of its dictionary comprises still images, not video clips, so they're a little tougher but not impossible to use.
Five ILU’s up!!! (those are “I Love You” signs beside each link) There are just so many great resources online for learning sign!