MythTV rocks! Couple it with a digital TV card, and it's something else.
Makes TiVo look rather lame (plus they're not available in the UK any longer), and walks over all the other DVR/PVR's I've looked at. Supports multiple TV cards, distributed architecture, picture in picture, HDTV, EPG, MP3 jukebox, MAME support, xsnes support, local weather forecast, DVD player/ripper and more! There's even a Webserver, so you can tell it to record something where ever you might be!
The distributed architecture is cool though. One backend server can feed multiple front-ends (which can even be an XBox, as someone's ported the front end to the XBox). That way, only the backend server needs to have the TV/Satellite card(s), and can stream over ethernet.
The front end doesn't even need a hard disk! There's a small version for VIA EPIA boards with the CLE266 chipset, so they can boot off the network and act as a frontend.