iMon-remote
I use XBMC on my HTPC, and it is great. I'm using a case that has a built in iMon IR receiver and vfd readout, which also lets me turn the system on from off (which isn't common with lots of PC remote controls). XBMC needs to be set up to work with these.

As my HTPC runs on linux (Arch linux to be precise), this first of all means setting up lirc and lcdproc. My case also has a volume knob and some media control buttons on the front that need setting up too. There are plenty of guides out there on how to set these up, so I'm not going to here - I'm just going to give my config files that might be useful for others.