1. Merriam-Webster's Online Dictionary, 11th Edition (it's the online version of the popular M-W Collegiate)

2. The American Heritage® Dictionary of the English Language (or, AHD4) [I really dislike all the advertising and the popups, which often cause my windows to shrink]

3-1008. (nearly) everything else, in no particular order

1009. Urban Dictionary, the most useless user-developed site of all