I run a multimedia living-room box on linux (used to be xubuntu, now I am running kubuntu). Here are some things I play around with.
Update 2011: I ended up just using a normal pc with XBC and kubuntu - that worked much better and I could hide the case behind the couch. Now it's almost "invisible"
MythTV
Power Saving
This is about my living-room box. I want to be as quiet as possible, and almost-instant-o (it mustn tkae longer than the projector to be ready). An important part is: power-saving
Ramdisk and Kubuntu
Since I want my box to be power efficient, I decided to reduce hard-drive access. One easy way to do that is to use ramfs for quite some things.