Boxee is a very complete media player and media hub. It was designed to let you browse your entire video and music collection with ease and to allow you to watch content both on your computer and stream it from various sources to your computer. The application has a very polished design, which looks great on high-definition displays. It was designed to be used on HTPCs in the living room, with big displays. Thus, you will see that the interface has huge buttons and you can browse it using your mouse, keyboard, or even a remote control.
Boxee is also a huge social hub. To use it, you have to create an account with the Boxee website, which you will use to log-in. You don't need an Internet connection for offline video playing, though. You can use your Boxee account to share what you have been viewing with friends. There is also support for apps which allow you to stream content from several video websites, like YouTube, NBA, Hulu, etc.
I have been following Boxee for a while, and, in my opinion, it is the premier HTPC application. It has extensive format support. It plays them all well and it has lots and lots of options.