I used the Mac version of Boxee recently. It looked pretty slick, with a nice, smooth UI - very similar to the original Joost client. It crashed on me a couple of times, so I guess I will wait for a more stable version. This was on the Alpha version.

I am using the windows version of Boxee which is still under alpha testing and it has turned out to be THE media-center for my laptop/PC. Easy, intuitive and extremely user friendly it makes viewing your online and offline media a pure breezy pleasure. I have stopped using any media center on my PC and have also reduced my usage of the VLC media player because of Boxee. Hope the windows version comes out to public early. This is one app you would not avoid missing from your PC.

A new media center application for the Mac (and soon to be Linux and Windows). Based on the open source media center application Xbox Media Center, aka XBMC, which we've covered from it's humble beginnings on the Xbox classic to its move to your Mac and your thumb drive, Boxee completely reskins XBMC and adds a new social element. With Boxee, your local content seamlessly integrates with web content from sites like Last.fm and YouTube, and Boxee shares what you're watching, listening to, and liking with all of your friends.

Boxee is building the ultimate social media center which I believe can change the way people consume and share digital content at home. Boxee allows users to easily consume and share video, photos, and music from various sources on their computers and TVs, interact with their friends around this content, and more. We think this is a huge leap from anything that's out there today.

Boxee just came out with their alpha for mac. I encourage you to sign up for their alpha plan, download, and get ready for an entirely new and exciting media center experience.