Cortex Command is free on Steam to promote the same dev's soon to be released out of early access game Planetoid Pioneers (with a 10% discount to owners of CC). The time span for this free offer to install is "just this weekend" no hours listed.
Want a parody of the Battle Royal game genre for free on Steam? Totally Accurate Battlegrounds (a joke game made for April Fool's Day) is free for the first 100 hours before launching at it's $4.99 price point.
Dirt Rally is free to keep on Steam for the under the next 48 hours. The theory is the game is going to be delisted. Also the game may not show up in your library even if you claim it, so check and try again later if need be.