An interesting one might be Reddit, although OSS isn't their formost product, their site is 100% open source (on GitHub). I think that must have posed some very

An interesting one might be Reddit, although OSS isn't their formost product, their site is 100% open source (on GitHub). I think that must have posed some very interesting challenges for the devs, I'd be interested to know how/if it improved their coding standards and security, if you're going to let the whole world look at your source code, you need to be confident there isn't anything in there you wouldn't want the world to see.