The sites I use regularly are ASLPro.com, Lifeprint.com, and SigningSavvy.com, though Signing Savvy will only let you look up 5 signs a day without a paid membership. The ASL dictionary I've had throughout my program is this (below) and it has been a great resource. Of course, a Deaf person will always be the best ASL dictionary. Good luck!
Sorry, the "link" feature here stopped working for me months ago, dunno why.