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.
OpenClonk 5 extended the game by adding new missions, and starting to connect these with a storyline ("the hero is sent all over the world of OpenClonk in a great endeavour to kill the evil king").
To reinforce this new approach, scenarios now have "achievements" which you can win by finishing them at a higher difficulty level, or uncovering secrets.
There's even a new league system. "Compete online for ranks and play against the best of the best. Just press the league button when hosting a game to sign it up to the league server, then use your forum username and password to join. If you prefer cooperative play, the settlement and adventure league are also soon to be opened."
Individual games have more detail in some areas. Go underwater and you'll now find coral and fish ("both peaceful and evil", apparently), while smoke graphics have been improved.
All of this is now accompanied by music, including three original pieces. Sounds good to us, but if you disapprove you can toggle it on and off by pressing F3.