Well, it's not a game, but it's made by Construct so I think it still counts. I was creating a BattleTech themed scrolling shooter when somehow it turned into... this. This a is screensaver of a Commando class Battle Mech stomping through a field. In the download is two versions, Peaceful and Battle. Battle requires a more powerful system, and I find that the Peaceful version is a better screensaver anyway.
To install, right click on the version you want and click "Install". This will automatically load it as your current screensaver and the Properties box should pop up. The only thing I can't make with Construct is the Settings menu of the screensaver. Because of that, the Screensaver will pop up every time you open the screensaver tab instead of in the preview box, but I don't find that much of a problem. Maybe it will be worthy of your screen?
I cannot take full credit, or really any credit for the model of the Mech, as it is in fact a 3D Cad Lego creation, by Primus at http://www.brickcommander.com/BC-CBT.htm. His models are absolutely amazing. All I did was download the model and render it for use in Construct.
WARNING: The Battle version requires large amounts of memory. Most modern desktop computers should be fine, but laptops, for instance, may not. It seems that a computer that struggles to keep up with the screen saver will eventually start displaying all the objects as squares and rectangles, and finally the app will crash.