Lix
www.lixgame.com
Lix, free and open-source Lemmings clone with multiplayer, an editor, and singleplayer puzzles

From website:

Lix is an action-puzzle game inspired by Lemmings (DMA Design, 1991) for Windows, Linux, and Mac. It’s free and open-source software, placed in the public domain via CC0.

Solve over 700 singleplayer puzzles. Play real-time multiplayer. Design your own levels.