This freebie has expired.
Humble Bundle is giving away X-COM: UFO Defense (a.k.a. UFO: Enemy Unknown). You need a Steam account to claim it as there is no DRM-free option. I do wonder whether after installing it, you can just back-up all the files and transfer to another PC. Some old DOS games were simple like that.
If you want it, you must be quick. At the time of writing, there is only 1 day and 6 hours left for this give away.