We do not impose the "not written by us" approach:
We do have some preferences when we deliver components that we have to maintain through a support contract or when the urgency to deliver is high and the schedule is tight.
We promote efficiency, not evangelism and dogmatic approaches. If we cannot demonstrate a huge gain, we will keep our mouth shut.
Functional languages have our preference.
We will propose Clojure most of the time either in the JVM, CLR or in Web GUIs (ClojureScript) when appropriate. We consider that this functional language is the most concise and flexible to rapidly create software components or solve integration problems. It also interfaces with other language JVM/.Net/JS libraries easily to allow reuse of your existing code base in some form or another.
We favour open source software in every way, operating system, languages, databases, ...
We do use commercially available software were it makes sense ROI wise.