http://phabricator.org/ - at Facebook

http://www.google.com/analytics/ at Urchin

I think pretty much the entirety of AWS would qualify. - https://plus.google.com/+RipRowan/posts/eVeouesvaVX

I have heard from friends at Amazon that that is in large part an origin myth. This is hearsay, so take it fwiw.

Nope: http://www.quora.com/Amazon/How-and-why-did-Amazon-get-into-...

In gaming: all of the idTech engines, the Unreal engine, and so forth.

Unix was an internal tool at Bell Labs, as was C.

Rails was pretty much just a framework over at 37signals before it spread.

~

This happens a lot in software.