http://www.mysettopbox.tv/knoppmyth.html
This is the easiest way to get Myth. Burn the knoppmyth ISO to a CD and it installs the linux operating system with Myth already built into it. Of course some linux knowledge is required.
Not only can Myth do everything Tivo can (although Tivo's interface blows all the rest away) it can manage and play your videos and music, give you access to news feeds, weather, internet and connect to your netflix account.