I've recently repurposed an old machine that I had laying around (custom built several years ago with miscellaneous parts) as a simple media center with an Ubuntu 8.10 operating system and Boxee as its front end, and I absolutely love it. I'm particularly intrigued by the custom applications that are possible, and I wanted to start a running list of recommended apps for it.

So, my question is this: what Boxee apps do you recommend, and why? One app per answer, please.