City of Steam. Starting at 2 a.m. EDT, the game will be taken offline for four to six hours while Mechanist Games pushes live patch 1.2: The Vault Unsealed.
This new patch adds quite a bit of content to the still-in-development City of Steam, including new quests, the return of the Riven and Draug as playable races, the addition of a new content hub by the name of The Vault and a brand-new Wilderness level known as The Broken Stair. The level cap has also been raised up to 32, and PvP enthusiasts will find a new map and new tournament event in which to slay their fellow players.
Mechanist is promising more details once the servers come back online; in the meantime, you can discuss the patch over at the official City of Steam forums.