What's possibly the most used piece of software developed at Microsoft? The WiX toolset. It is used by Windows, Office, and even MySQL, among many other products, to install. What's even better is it's done by an all-volunteer team and it's open source too. I videoed the team a while back and just put the video up. Rob Mensching of the team is here at MindCamp.
Speaking of MindCamp, I am uploading an occassional photo on my Flickr feed.