I've been keeping an eye on this one for a while now as one of my last few hopes for a trip back to the days of classic UO.
Now I don't know how this little gem has been avoiding discovery and keeping its hype minimal so far, but I give you:
I bring it up now, because this game has made some very notable headway since I first discovered it. Its in closed beta at the moment, current testers have limited invites and every so often they open up sign ups on their main page for new testers.
Its a sandbox game, but a properly fleshed out sandbox with toys and freedom similar to that of classic UO, not like some of the half ass attempts we often see where apparently removing a bunch of stuff out of a MMO and not putting any toys back in == sandbox.
Beyond that, it has its own "twist" on the genre, further pushing the idea of user created content in a sandbox game. You can get a "realm", basically your own piece of the world that you can shape and build with the many toys and tools available in game, which gets "linked" to the many other player and developer created realms in the game. Thus the game's name, Linkrealms.
This one should definitely grab the attention of UO vets.