Pingus is a free Lemmings-like game covered under the GNU GPL. It features currently 22 playable levels and runs under a wide variety of operating systems (Linux, Windows, etc.)
Pingus has started at the end of 1998 with the simple goal to create a Free (as in freedom, not as in free beer) Lemmingstm clone. The Free means you get the sources and are allowed and encouraged to change and distribute them. You basically get everything that I used to create this game. In the years it has grown than quite a bit over the original goal and become something more than just a clone, we have original artwork, a built-in level editor, new actions, multiplayer and a few other features.
If you don't know Lemmingstm, here comes a short introduction. Its a puzzle game developed in 1991 by DMA Design. The player takes command in the game of a bunch of small animals and has to guide them around in levels. Since the animals walk on their own, the player can only influence them by giving them commands, like build a bridge, dig a hole or redirect all animals in the other direction. The goal of each level is to reach the exit, for fix multiple combination of commands are necessary.