Any successful examples of a software dev firm building reusable dev tools?

I'm interested in examples where companies that develop custom software for clients. Every project is different, but when you focus on a particular area there are common components. What's the best model: open source, proprietary or something else? What about QA, training, documentation, etc. is different for a "tools" project than a normal client engagement. How do you make a project like this awesome for developers.

Thanks in advance for any thoughts!