World of WarCraft: Obviously, the most popular one of the bunch and very Mac-friendly. Both Mac and PC users play WoW on the same server. Blizzard did an excellent job of coding the UI so all AddOns work on both platforms. It's very forgiving on system specs; I can run it for the most part on my Macbook. One advantage the Mac has is the in-game movie maker that'll let you record in-game action to a quicktime file. This file can then be imported to iMovie for post-production.
EverQuest Macintosh Edition: I wrote about this a while back. It's a nice, although low-pop alternative to WoW. It's old-school EQ, with corpse-runs, experience loss (and potential level loss) on death -- all the stuff we called fun back in 1999. It lives in SOE's Land That Time Forgot, though; the last patch was several years ago. On the plus side, that means less nerfs. I've played it and the performance is pretty good. The graphics are dated, but given the meager offerings for the Mac it may not be a big deal. While the community is small, it's also tight-nit, with an active forum community on eqmac.com. This is definitely a game I wish I had more time for, and wished SOE paid more attention to.
Minions of Mirth: Minions is an interesting hybrid of a game. It's got an online and offline mode, as well as a free and a paid (premium) version, which will let you host your own server, if you want. There's a one-time payment of $29.99 with no monthly fees. There's an imminent expansion pack due out in April that will be free to all premium members (and will also be included as part of any future premium accounts). You can create a party on the single-player version, but it will only be represented by one avatar, which takes a little getting used to. I reviewed it a few years ago for PC Gamer (and, full disclosure, it didn't fare well), but it's been on my list to take a Mac-only look at. There's a vibrant community on its forums, so the game is still holding interest several years after launch.
Lineage: This one I haven't played, and don't know much about. My only experience with the series is in the Lineage 2 beta on the PC, most of which was spent screaming, "No, no, make it stop!" Lineage, the prequel, natch, is an isometric-view that could take some getting used to in today's 3D world. Mac and PC players play on the same server. If I read the website correctly, there's a seven-day trial period. That sentence also uses the word "purchase," so I'm not sure if there's a translation issue, or if they hit you with a fee for the trial.
EVE Online: Fair warning, this one's in the "I haven't tried it much, but really should" section. EVE's getting quite a name for itself with decent growth over the last few years. It gets bonus points for having a Linux version as well. It's a space-themed MMO, with a PvP focus. One word of warning: it's got the steepest system specs of the bunch. You can't run it on a PPC computer; it requires one of the newer Intel-chipped models, and doesn't support the Macbook either (although it did run on mine). From my initial playtime, as well as testimonials from friends, there's a healthy learning curve to EVE.
Vendetta Online: Another space-themed one, and unlike EVE, it'll run on darn near anything. Vendetta is also very PvP based-outside your home area, you enter the gray zones where you can be attacked. It also uses twitch-based combat, which is nice to see. The last time I looked at Vendetta was almost three years ago (and it didn't fare well then), but I don't know what improvements have been made over the years. There is a limited trial (about eight hours) but there's no client cost, just the monthly fee of about $9/mo.
World War 2 Online: I'm going to mention WW2OL because there's a Mac client for it, but when someone mentioned it to me, it immediately made me think: this game is still around? I'm glad to be proven wrong and it's just me having a "Spinal Tap: Where are they now" moment.
Can't Really Put a Category On it-themed
Second Life: Let's not get into the "is Second Life really an MMO" argument just yet. In my book, lots of players + persistent world + Mac-native client = inclusion in this column. Second Life is definitely more virtual world than game. There's no levels, classes, grouping etc.; instead you create an avatar and just explore the world. All the content is user-created which has its good and bad. It's good because some people can do some amazing things with the creation tool. It's bad because, well, some people do amazingly ugly things with the tools. Also, that spot you really like might suddenly disappear because the owner got sick of it or couldn't afford the land anymore. There's no fee to play Second Life, but you can upgrade to a premium account for a base of $9/mo that'll let you own virtual land. That monthly cost, though, scales with how much land you own. SL also actively embraces the real-money transaction model: you can buy the in-game currency, Lindens, with cash, or convert Lindens to cash. Under Tiger, I got decent performance on my Macbook. With Leopard, the performance became abysmal, but, with the 10.5.2 patch & graphics upgrade my performance got a lot better. I've seen reports of people having issues with certain models of Macbook Pros as well.
These are the MMOs with Mac clients I'm aware of. Which ones have you had any experience with, and what ones did I miss?