I hacked my Series 1 Tivo about 2 years ago and added a 100 Gig HD and a wireless nic card.
Once you install the drive and the nic card, you've already started installing things like Telnet, Shells & FTP. I installed that script that lets you completly control your Tivo remotely and it's pretty cool.
I also added the tool that enables you to start up a server so you can extract all the video & audio and burn it to DVD.
One thing I found while I was snooping around was the logs that Tivo keeps of all your viewing habits. That truly pissed me off. Every fucking button you hit on the remote and channel you watch it logged and sent to them. I ended up setting a cron to delete the logs every few minutes.
The forum that I ended up finding that was most useful for files and support was located at:
http://www.dealdatabase.com/forum/index.php
It's a fun little hobby.