Dice.bot

[Kublai]

I've been using this dice.bot for good effect so far:
http://www.irony.com/webdice.html

[jc_madden]

Yes I've used it it's good though it's royally SCREWING me right now...

[Verrain]

Best one out there in my opinion.

http://www.openroleplaying.org/tools/dieroller/

is very good too for those times when Irony is down.

[Yagathai]

TCRgames has a pretty decent one, IIRC.