Pingus is a free clone of the classic Amiga computer game, Lemmings - only this time, you're guiding penguins.
The basic idea is a simple one. Each level opens at a landscape with increasing levels of danger. Cute penguins then appear, and start marching across the screen. Left alone, they will usually fall prey to some horrible fate, for example falling to their deaths off a steep cliff, or walking into a booby trap (these penguins aren't bright). Your task is to guide them safely to the exit, so they can progress on to the next level.
Initially this all seems very easy. You might just click on a penguin to start it digging, for instance, and finding its way to a lower level and the exit. Or click again to give your penguin a backpack with a propeller, so it floats gently down the cliffside, rather than plummeting to a nasty end.
As the levels progress, though, so the challenges become greater. The landscapes are more complex, the abilities you can give each penguin increase, and keeping your charges alive takes serious concentration. Expect extreme frustration on a frequent basis, but also regular triumphs when you figure out how to overcome a particularly difficult problem and manage to move to the next level.