Just thought I'd mention CraftStudio (http://craftstud.io/), the multiplayer game-making platform I'm working on. You could say it's a Minecraft-style engine but you get to make your own models, paint your own map blocks and (soon) write your own game rules (so you'll be able to make any kind of games, not just first person stuff).

Video of CraftStudio in action: http://www.youtube.com/watch?v=r1QbJO3y_O0

Hacker News thread from yesterday: http://news.ycombinator.com/item?id=3751455