@dajbelshaw In 2006 I worked at a company which was largely FLOSS, both the office systems and the products.

At the last company I worked at on day 0 I was issued with a laptop and the CEO said "You can install any OS you like, except Windows. The only requirement is full disk encryption". As far as I know there was one Windows laptop in the company and everyone else was running mostly Debian.

So I don't think I've worked at any companies which were entirely FLOSS, but I have worked at a couple where the amount of proprietary software was small.