DiceBot
A rather good custom designed dicebot for RPG games - full details on the webpage.
IRC Dice Rollers
This site contains three IRC dice rollers for use with mIRC. There is a "Standard Dice Roller" for general use and two more each designed for use with a particular system. Included is D&D3e and Alternity.
Wormspeakers Web Pit
Java based dice rolling and map drawing software. Can be used to support IRC based roleplaying.