Some tools lend themselves to starting small and expanding through add-ons, pro features or alternative versions. The example that comes to my mind is GreenSock (http://greensock.com) which started as small, simple Flash (ActionScript) library, but evolved into a large set of animation related libraries and plug-ins covering also HTML5 (JavaScript) and offering various licences starting from free to commercial/paid.

I don't know the internal details, but I have used it on multiple occasions over the years and the impression I get is that it evolved from a free, personal side project into more professional product and company. I suspect that the creator also gets commissioned projects as additional revenue stream. And judging from its consistent evolution over the years it must bring profits that justify working on it!