Three of four Steam World games are on sale right now.
The Steam World Heist sale is especially steep (6 bucks, on sale from 20). It's a little like X-COM but with some big twists. One big difference: the game is presented as a 2D sidescroller rather than 3/4 perspective. Another big difference: attacking requires manual aiming. Your leader's first weapon has a laser sight for training, but other weapons don't have it (so far; haven't been playing long). Manual aiming let's you get head shots, carefully shoot just over cover and even ricochet bullets. Unlike X-COM, if you die, you lose half your money, but everyone is revived. It has 5 difficulty levels and defaults to the 3rd one, but you can change difficulty on a per mission basis. The game recommends increasing to the 4th difficulty if you want a proper challenge (which I have done). I'm having a lot of fun with this so far.
The DIG games are also on sale (3 down from 10; and 10 down from 20). These are metrovania (rygarmania, etc.) games with a digging mechanic. It was a turn-off at first, but digging ended up being what makes the games unique and fun. The level design in the first game is very limited, while the second has a fully realized world and is easily twice as long.