The main open-source media centre software for the Windows platform, MediaPortal is a serious challenger to Windows MCE. For those of you who don't understand the media centre concept, basically the idea is that all the content and power on your computer should be available on the big screen. So via what's termed a 10-foot interface by Microsoft, essentially an interface designed to be used from about 10-foot away, all your music, videos, radio streams, RSS news feeds, weather and much more is available at your fingertips. On top of that, the power of the computer means, assuming you have a TV card installed, you can pause, rewind and fast-forward live TV as well as record and timer recordings using an electronic program guide.
It's something that you don't realise you needed before you use it, but once you do, you won't go back.
I'm active in the forums under the username samuel337. I have also contributed various patches to the software itself, including the party shuffle feature (now superseded by the new last.fm feature), as well as revamping the My Alarm plugin. I'm also the creator of the External Control Plugin and the MediaPortal web interface, more on that here.
I got involved in MediaPortal from early on, when it was still a young open-source project. It was at the time, the most promising free option after myHTPC evolved into Meedio (now bought by Yahoo!) and many of the other options being quite half-baked and lacking traction and support. I started out posting in the forums, but eventually there came a time where I needed something that was not available in MediaPortal - the party shuffle feature as seen in iTunes - so I bit the bullet and started hacking the source. MediaPortal is written in C#, so while I was doing that I also had to pick up C# as well, which turned out to be quite useful as knowing the two most popular .NET languages (VB.NET and C#) meant most of the .NET world was open to me.
From then on, I sorta grew attached to it, so I stuck with it. It is now a pretty big and fast-moving project, with some serious traction. Even now, there is no real challenger to it.