ZephDT

I know of some unique MMO's.
There's Maplestory, the 2d side scroller. Although there's no pvp and it's kind of a grind, it's undoubtedly one of a kind.

Cabal Online's combo system entertained me for a while. That's the only thing that really stood out, in my opinion. Still, it's a pretty popular game.

Dream Of Mirror Online(DOMO) has an interesting dual-class system, and the party quests are really fun. One of the best F2P MMORPG I've seen so far.

Silkroad Online's merchant/thief system was also pretty fun, although the game is pretty laggy.

Oh, and I can't forget Ragnarok Online. A lot of classes, and interesting gaming mechanics(Stat/skill mechanics, PvP, card system). The free server is filled with bots, so most people play on private servers(me included). Or you could play on the P2P servers. It's the only game I keep coming back to, and the best F2P game, in my opinion. PM me about this one if you need some more information.

Another game I keep coming back to is Tibia. Nostalgia might be the main reason, since it was my first MMO, but it's also pretty unique. Granted, the graphics and the community kinda suck, and it's 2D, but it's still worth some of your time, I'd say. The thing that appeals to me is the ability to kill one on sight(On PvP servers). You could also go on a non-PvP server, though. You can also PM me about this game. Don't let the bad graphics scare you away!

I nearly forgot to mention Rush On Seven Episodes(ROSE Online). ROSE online was published by Gravity(the developers of Ragnarok Online). It's a fun game, though I can't think of any redeeming qualities it possesses.

I'll put these games in order, the best game being the righternmost.
Cabal Online, ROSE Online, Silkroad Online, Maplestory, Tibia, DOMO Online, Ragnarok Online.
Tibia and Maplestory's spots can be switched. I just enjoyed Tibia more.
Keel in mind that these are just my opinions.

I've played most, if not all of the free MMORPGs up to 2008, and these are the ones that stood out most.