Try a
Ragnarok Online private server.
I would suggest a WoW private server, but every single WoW private server has a million bugs, to the point where it's not fun to play. Ragnarok private servers tend to be pretty stable.
Alternatively, you could pay $15 a month and get the official version of WoW, which is more or less the best MMO out there at the moment.
Lineage II is another good one that has decent free servers.