If you're trying to advertise the actual game there are plenty of mmorpg or just general gaming forums to advertise on, take your pick. Also, maybe contact other game sites, preferably RPG I suppose, and find advertising space on their sites with banners and whatnot.
The hardest thing i find is being able to contact other game sites, due to no one almost displaying contact forms or email address, making it so hard to get in touch with
We have been developing browser based mmorpgs long ago, I can suggest only AdWords and http://www.game-advertising-online.com/ and the biggest 2-3 toplists those have big traffic. If you do not have enough money ($10.000+) to advertise your game, you should be very lucky to make success.
Adwords will definitely help, but you can also try Yahoo Search marketing if you are little tight on your budget..