The Clonk series is fairly obscure, as such things go, but it has its own fanatical following -- and is notable for the huge variety and scope of its gameplay, as well as for its somewhat awkward controls.
Originally designed by Red Wolf Games, a German shareware developer, Clonk is a weird combination of aufbaustrategiespiel, arena combat in a destructible environment, and platformer. You control a clonk, a somewhat hobbit-like fellow, and run about a 2D, platformer-like environment, controlling with WASD keys; you have an inventory, and can, depending on scenario, obtain a huge variety of tools, including bows, swords, grappling hooks, magic spells, explosives, shovels, and so on. Almost everything in the environment is traversable or destructible with the right tools, and in platform levels, traversing is the goal.
However, many levels are based instead on resource extraction and construction of buildings to create tools and items from those resources. In some cases, up-building is the victory criterion, but in others, you use what you built to fight against other teams.
And, of course, there are battle scenarios where extraction and up-building are deprecated, with some tools available in chests, and the focus on arena-style multiplayer battle.
The controls are awkward as hell, and multiplayer is not advised until you've mastered them; but there's a huge variety of gameplay to explore. The Clonk games are the kind of thing that makes you realize anew that, yes, independent game development is where creativity is found.
Clonk Rage is the last game Red Wolf developed; it's shareware, but the demo contains quite a lot of content, and the earlier version, Clonk Endeavour, has been released as freeware. Red Wolf is now working on a 3D version of the series -- but a group of open source developers have decided to build on and extend the 2D game with OpenClonk, based on the Clonk Rage source with Red Wolf's blessings. Despite the name, OpenClonk isn't an open source clone, but instead has wholly new content and levels, with the best ones created by the community incorporated in the current build.