Cabal Online is a MMO that is very similar to World of Warcraft but it is free to play. i dont know if it is open source but i would like to see it be a portable app
Edit:
apparently it is open source so i would like to see it made into a portable app
I googled it and found it it is open source. I find it amazing how so many questions asked on PortableApps.com can be found in less than one minute on google. It looks like it could be make portable.