It's a media center software (based on XMBC) with a focus on online content with videos from Hulu, Netflix, Comedy Central, YouTube, torrents, etc.; pictures from PicasaWeb and Flickr, music from last.fm, NPR, etc.

Why should you use it? It's a great step towards combining online content with a TV. It has a lot of pieces that are missing from AppleTV. (even though it's possible to install Boxee on AppleTV :))