UPDATE: DX11 and adoption rate would HAVE to put Unigine at the top. Although I can’t say I know any MMO specific frameworks that (if i had the money) I would toss onto Unigine for integration.. meaning still a version of roll your own already in the marketplace.
On the other hand. Unreal Engine 3 has the Atlas MMO Framework already for integration. Their licensing cost is very strong, although I am fairly certain strong profits from any project in UDK that your team undertook would get you a lot of the way into the door of using UE3/Atlas (similar to what happened when the Source Engine did it’s thing the way of UDK).
My favorite game engine for developers who have a REAL budget is called the Vision Engine by a company called Trinigy. UPDATE: Also Unigine and now Unreal Engine 3
Unfortunately, when they don’t even list the price, that usually means it’s put of my price range. Nevertheless, I highly recommend the Vision Engine for small to medium sized developers interested in MMORPG (or other types of games across the various platforms) development. (still have to roll your own on the MMO side of things though.. no open mmo sdk/framework out there yet that is integration ready)
Random Useful GameDev things to look up on Wikipedia
Direct3D 11, MMORPG, Bullet Physics