XBMC, Now with Less XB!
Xbox Media Center (XBMC) is one of those projects whose name makes less and less sense as time goes on. Sure, people still are using XBMC on an actual Microsoft Xbox, but for the most part, XBMC now is run on computers. In fact, recent versions of XBMC installed on an ION-based nettop makes just about the perfect media center. Version 10 (Dharma) introduced a fancy plugin system that allows XBMC to be extended beyond its built-in media-playing abilities. The next version, currently in development, will focus partially on recording as well as playback.
When it comes to performance, it's hard to beat XBMC. It's faster and more responsive than a Boxee Box, has local media playback unlike the Roku, and is open source, unlike Microsoft's media center options. It does currently lack in premium on-line streaming, but that's largely because the live version is based on Linux. It's a trade-off I'm willing to make. (I actually keep a Roku for that purpose and use XBMC for everything else.)
Check out the latest features and download a copy for your operating system at http://xbmc.org.