OpenClonk is a free, open-source multiplayer version of RedWolf Design's "Clonk" game.
The core program gives you control over "Clonks", "small but nimble humanoid beings". You move them around a 2D horizontally scrolling world, deforming the landscape, setting up mining operations, managing your economy and taking part in "fast and fiddly melees".
While there's nothing too unusual about any of this (think Minecraft, Terraria, Worms or Settlers), OpenClonk does make it extremely easy to develop new content. Mape, for instance, is a dynamic map generator which creates maps that look different each time, while still containing whatever recurring patterns you choose (acid, coal, sky islands, whatever it might be).
The C4DT Eclipse plugin takes this even further by providing an extensible Java-based development environment. This helps you use Clonk's own C-like scripting language, C4Script, to create and test new scenarios.
Complicated? Well, yes, but there are plenty of tools to help you along. And of course you don't have to develop any extras yourself: if you prefer the easy life, you can always just download content created by others.