One of the bigger sites using this would be <a href="https://www.ruby-lang.org/en/" rel="nofollow">https://www.ruby-lang.org/en/</a>.
startssl.com have free certificates as well, <a href="http://www.startssl.com/?app=1" rel="nofollow">http://www.startssl.com/?app=1</a> and not only for open source projects.