Windows Media Center is free with Home Premium. XBMC is also free and acts as an open-source media center and runs on Windows, Mac, Linux, and XBOX systems, . Boxee is based on similar content as XBMC but brings you web content. Plex is a media center for Mac but is also a variable of XBMC.
I agree with XBMC. Its been around since the original xbox. It has had so many years to mature into a sweet system with lots of options built in. Works on pc (low end to high end), mac, linux and xbox. Running it on my custom htpc and i couldnt be happier. Keeps my media well organized and the plugins you can get for it, along with the ability to launch programs outside of xbmc makes it a must for htpc's.
Thanks for the replies from everyone. Getting more specific. I'm looking for the following:
1. Small box
2. HDMI compatible for video (don't have an HDMI supported audio receiver but possibly later)
3. low noise
4. I would like to record from HD antenna & possibily ROKU player (doing away with cable TV)
5. DVD RW player (Blueray not neccessary but would like)
6. I would like to be able to stream to other TV's or possibly the internet (Not sure if a NAS is required for this to work).
7. I would be able to build the system myself. Not sure on the best operating system for this. The XBMC based system with Boxee looks good.
You need to be more specific. Are you looking for a bluray/dvd surround sound system to go with your tv or something else. What are some features you are looking for?