RemoteDroid allows you to use the touchscreen and keyboard of your G1 as inputs for your PC. I’m trying to think of a situation in which this would be handy and not just cool, but I can’t. Of course, that doesn’t change the fact that it’s totally awesome. Actually, it could be useful for a home theater PC setup, or for making people think your computer is haunted.
You’ll need to install the “server” on your Windows machine, and then of course the app on your G1. After that you’re only limited by your imagination. And some other stuff, probably.