Crystal Crisis by Nicalis is currently free on Steam - it normally costs €16.79.

Crystal Crisis is a puzzle battle game that combines fighting-game presentation with falling crystal mechanics: in head-to-head matches, players clear crystals off the screen to attack their opponent. The roster includes 20 playable characters, among them crossover guests like Quote from Cave Story, Isaac from The Binding of Isaac, and Astro Boy.

The offer only runs until 21 September, 11:59pm, after which the game stays permanently in your library once claimed.

A color matching, head-to-head battle game! Crystal Crisis pits you against a friend (or five!) in an intense battle of wits and reflexes! Clear crystals to send junk blocks to your opponents and knock them out of the game. Utilize the innovative wraparound mechanic to move crystals off one side of the screen and onto the other! Compete against a huge cast of 20 characters, including crossover characters like Quote from Cave Story, Isaac from The Binding of Isaac, and Astro Boy! Unleash devastating Burst attacks - 40 unique special moves in total - to turn the tide of battle! Enjoy a huge assortment of single-player modes, plus local and online multiplayer!

Source: Steam