Website Tech Tips For Us has a really nice list of 21 lesser know but highly useful open-source applications for Windows, worth browsing for new finds.
21 Awesome (But Lesser-Known) Open-Source Applications for Windows (Tech Tips For Us)
These applications range from moderately popular to downright obscure, but all of them are open-source and FREE. All of them are worth the install time if you have never tried them. As a side bonus, many of them are cross-platform as well.