Although we use MS for our core we do use Linux and open source projects as part of our stack. The routers/firewalls, redis for caching, and haproxy for load balancing are all run on Linux. In addition we use Linux for a lot of the management such as Bacula for backups and Nagios for monitoring. We try to be open about our infrastructure and both George and I blog about the changes we make at http://blog.serverfault.com