The main free RPG is Second Life
http://secondlife.com/community/linux-alpha.php
Is not in beta yet, but the basic is free and you can earn money to do stuff.
Ultima Iris is a other. You have free basic, but the game is kinda worthless
with out some cash to buy stuff.
http://tinyurl.com/36xjex
Here is some more in this link
www.linuxlinks.com/Software/Games/Role_Play/
Most of these games are a tar ball file and you need to build from source.
Same here
http://www.happypenguin.org/
Here some games that run on Ubie
https://help.ubuntu.com/community/Games
If you do not have a high graphics card. Your choices are going to be even
smaller.
http://packages.ubuntu.com/feisty/games/