Open source on-line roleplaying application. It is an IRC-like application written in Python to facilitate roleplaying on the Internet. In addition to having an advanced dieroller, it also has a map, a node tree (often used for character sheets), and a standalone server. Being Open Source, you can also extend both the server and the client to give them additional capabilities, though I don't know of anyone but me who has modified the 'official' server code outside of the actual development branch.
A number of Exalted campaigns currently run on it (at least a dozen, by my count).
One of mine does. It works very well. On the other hand, if you need an easier to use map and don't care so much about the rest, Gametable is good too.