EVE online is something you can run without a susbscription, buts a MMORPG. You need a lot of email adresses and you'll restart your char every 14 days, but ships and stuff can be saved in a corporation hangar. (The WS people have a corp going, some fun.)
Americas Army used to be fun, but the latest patch won't install, and I stopped bothering soon after that, so I can't judge.
FS2, well, if you can find any people on....
CS Source, if you don't mind the chat or the idiot voicecomm, there are some fun servers using nippermaps, but if you know nippermaps, that's no serious game.
If you want an RPG, check out Kingdom of Loathing, but I think you already did.