Microsoft, as part of an XBOX 360 update introduced "Games on Demand" which allows users to search through XBOX and XBOX 360 games, purchase them and download them to their XBOX 360's hard drive. This week, they've introduced the same functionality with their Games for Windows service.
When users sign on to Games for Windows and downloads the client, they can then search through a library of games and download them to their PC. One of the games, Tinker, is available for free (provided that you just download and install the Games for Windows client).
Here's a tip: If you use the same Windows Live ID for both Games for Windows and XBOX Live, you will not only get to share the account between both services (use one login for both), but you can add any gamerscore you get from Games for Windows games (including Tinker) to the gamerscore you achieve in games for XBOX 360 and XBOX Live.
Have fun, you crazy gamers!