You could wait until Thursday morning to get one of the best turn-based strategy games that's ever been made. Alternatively, you could go to Humble Bundle right now and get X-COM: UFO Defense for free.
You can grab the MicroProse classic for the next day and a half for nothing, although you'll be getting a Steam version of the game instead of DRM-free version.
Once you've dissected the original X-COM experience, I'd recommend going to the OpenXCOM website and exploring some of the creations there. The "Final Mod Pack", a collection of 70 community creations, is a good place to start, but there's a range of smaller mods if you just want to tweak individual elements.