I noticed recently that my Tivo started displaying a new menu item: "Enable Home Network Applications". Encouraged by the implications, I did a a little research and I ran across Galleon at SourceForge. I downloaded and installed it, and I have to say, it's ever so nice.
Despite a clunky interface typical of open source (and of Java apps), I didn't have too much trouble figuring out how to set up true two-way integration between my PC and the Tivo. I can now move shows back and forth, controlling the process from either the Tivo or my PC. This basically allows me to use the PC as both an archive/backup and a second TV. You can even set up rules so that certain shows are automatically downloaded to the PC.
Of course, I'm only scratching the surface of Galleon. It's really more a Java-based platform for writing custom Tivo apps than it is a transfer program. I've played around a bit with a few of the plugins that ship with the program, but haven't fully plumbed the possibilities. So far, my favorite is the podcast plugin. I like being able to listen to any episode of Science Friday in the living room on demand - it brings the Tivo experience to radio.