Depends on what kind of dictionaries you like, and what you use them for.
http://dic.yahoo.co.jp is good for most things, though I like
http://kotobank.jp more recently, especially for specialized jargon-type stuff.
http://www.weblio.jp is pretty good too though I haven't used it as much. Sanseido is good for quick definitions, though I find I remember the more detailed ones more. I mainly just use EPWING dictionaries now, so I don't know about any new sites that may have popped up, but hopefully something in this post might be helpful. ^-^
Edited: 2011-11-14, 3:44 pm