Everyone uses open source software, there's some BSD-licensed stuff in Windows IIRC.
As for more grounded statistics:
http://news.netcraft.com/archives/2011/10/06/october-2011-web-server-survey.html
http://distrowatch.com/stats.php?section=popularity
http://www.webmasterpro.de/portal/news/2010/02/05/international-openoffice-market-shares.html
http://sourceforge.net/top/
http://www.w3counter.com/globalstats.php
http://top500.org/stats/list/37/os
http://www.mysql.com/why-mysql/marketshare/
and just going to the project and looking up the number of downloads