This is interesting though not totally surprising:

[O]pen source software is not only available but also more popular on Windows. Note that of the top 25, there were 16 projects available only for Windows and none that were available only for Linux.

http://www.vinodunny.com/blog/post/OSS-for-Windows–SourceForge.aspx

This definitely matches my anecdotal experience. I have rarely looked for something on SourceForge that wasn’t available for Windows.