I was all set to do a media centre with Geexbox but it turns out you cant record tv with it so whether you could call it a rival... No contest imo.
http://geexbox.org/en/index.html
Support isnt bad though the remote control has odd actions for some buttons.
Get a load of the system requirements, I was going to do one with spare parts but now, with XP MCE, its a whole new build.
Its a live cd (boots from the cd then ejects it) but it can be installed on a hdd. I think its 6mb! Its just that record thing.. you can only use one tv card at once too.