The Steam summer sale begins early and, immediately, there's a whopper of a deal. I love Sins of a Solar Empire: Rebellion. At the moment it's my favorite game. The base game is currently $7.99 ($25 for a 4-pack) and the two DLC offerings are $2.49 a piece. If you like space opera and 4x strategy, you cannot do any better than this (although some prefer Distant Worlds: Universe). You remember that part in Return of the Jedi when Luke looks out the window and sees the mind-bogglingly massive "last stand" battle taking place in outer space? Sins is a little like that, only bigger, if you want.
Even if you're familiar with games like this, I suggest slogging through the six tutorials, which take around five minutes a piece, before going head to head with an easy A.I. in a tiny map. A "tiny" map, by the way, has more than twenty planets to conquer and can take more than a day depending on your style of play. This is the game that'll be holding me over until Civilization: Beyond Earth arrives.