Open Source Windows has a pretty neat list of free open-source software for Windows grouped into various categories. I would say it’s a good place to start for those who are searching for an open-source/free alternative to the software they are currently using as they have handpicked the cream of the crop for each category.
While on open-source, I came across this page which has a list of open-source games for Windows. Haven’t tried any of them though. ;)