Well, I\'m playing Neverwinter nights right now. It costs about 30$, but after you\'ve bought it you can play it all you want over the net. I\'m not sure you can call it a MMORPG since the servers (uses gamespy) takes like 100 PCs max (it\'s something similar to Diablo II, but a lot better), but the feeling is there. There is unlimited types of weapons, ammo, armors, worlds etc. because the game includes an editor (which is the most powerful and easiest to use editor I\'ve ever used) where you can create all kinds of items with their own unique attributes.
It\'s also one of the few games where there is RP (Roleplaying) servers. There is actually servers for all kinds of players.
You can play it in single-player as well and download an unlimited number of mods. the whole world can be manipulated with so-called \"HAK-paks\" allowing you to add your own sound-FX, music, textures etc. into the game. There is a lot of people who loves this game so there is a LOT (and I mean a LOT) of modules you can play, both MP and SP.
One of the best things about the game is that you can become a dungeon-master in a module you, or some other guy have created and create missions for other players directly in game!
For example, just like in Dungeon Keeper you can control NPCs or monsters whenever you want, add monsters, surprise PCs etc.
There is a lot of other stuff that you can do as well...
If you don\'t know about this game you have missed something.
I strongly recommend it!
http://nwn.bioware.com/