I just found this and thought it sounded pretty cool. Haven't had a chance to play it yet, but it runs on Linux, BeOS, Windows 95/98/NT/2000, MacOS, Solaris, Irix, and AtheOS (AtheOS was on /. not long ago - http://www.atheos.cx/). 

Arianne (http://www.arianne.cx/) is a virtual world. It's a massively multiplayer role playing game you access through an Internet connection. You choose your own virtual life and immerse yourself into a huge, wild new world, where you have an unprecedented degree of freedom to shape your own destiny. We try to get a full gaming experience through Open Source, you can help us improve it. We are planning a technology that will allow better PING that other online games, and also a good game experience if PING goes up. We are creating such a huge land that you will need several lives to cross the world from point to point. Non player characters will have a very important place in Arianne as we will try to give them real lives and personalities. We don't encourage player killing but if it happens only in-game rules, for example the justice system of a kingdom, can punish it. We are now supporting Linux, Windows, BeOS and MacOS and we will increase the number of supported OS in the future. A unique role playing system, with simple, easy to learn rules and a great freedom of action, you can do almost whatever you imagine. Huge amount of objects, weapons, spells, races, clothes, ...

"Linux is for people who hate Windows. BSD is for 
people that love unix."