Endless Space Free to Play this weekend...
..on Steam, and half price if you buy it after that.
I couldn't find the Cheap Games thread so this will have to do.
I spent all of last weekend playing this game and it is really excellent...up to a point. It is easy to jump right in, nothing is hidden from you, the interface is a joy to use, and all in all it is an interesting little 4X.
The point I mention is about 25 hours in, and for me it was once I found the optimum path through the tech tree, I got a little bored. However if that point is 25 hours for you, it is unlikely that you'll hit it in a single weekend, so please go and try it out and have fun.
The game is on my GOTY list for sure.