Guild wars is probably the best designed online game without subscriptions, but it isn't completely free. When Diablo III comes out, that might be a decent match. Other than that, though, I can't really think of anything. Most free MMOs are pretty similar: they all require huge time investments to get decent at the game and have much fun.
I'd suggest just going through this list and looking at screenshots of any game that sounds potentially interesting:
http://www.mmorpg.com/gamelist.cfm
One that I can think of offhand that was entertaining for a while is Dungeon Runners (made by ncsoft, the people that make Guild Wars). It is different enough from the normal grinding MMOs to be fun for quite a while before its own type of repetitiveness starts to kick in.
If you're playing with a group of people, I would also recommend looking for a newly released private server of RF Online. That game can be tons of fun, though servers have a tendency to become unbalanced after a while (mostly due to corrupt GMs) :/