First-person combat game Chivalry: Medieval Warfare is currently available for free via Steam for Linux.
Torn Banner Studios has made the game free for a day to promote their upcoming game Mirage: Arcane Warfare, whereby new users get 10% discount on pre-orders. Unfortunately there is no word on Linux support for Mirage: Arcane Warfare.
You can add the game to your library from here.
Check out gameplay video: