A Spoken Sanskrit Dictionary (English - Sanskrit and Sanskrit – English). The most useful feature is the possibility to look up words without using the diacritical marks. It is though less reliable for Classical Sanskrit.
Monier Williams on line, the most famous Sanskrit Dictionary ONLINE!
A dictionary that gives the possibility of looking up words with endings. It is though not so easy to understand how it works, especially when you need to transliterate the diacritical marks. However it is a very useful tool.