I spent tons of time with Google Search Appliance (at least 100 hours reverse-engineering it) it was just a CentOS machine with a daemon called Babysitter (which was just a loop restarting services), and a C++ binary called gws (Google Web Server).

Fun fact, if you ran gws without its config files you would see the real front end for Google Search, News, etc.

Web configuration interface was in Java, writing some XML templates if I remember well.

So taking all of that, besides a very boring OS there was "nothing" or very little amount of open-source they were using.

It was more all homemade (except the OS).

Fun fact: There was a secret hardcoded password in clear (but only for physical access).

EDIT: Password was different for each instance, not the same as I thought.