Well, I'm a web developer, so I rolled my own. I built a reusable framework for SaaS projects with just the kind of features you mention, and I've been using it both for myself as well for freelance clients.
Basically it's a PHP framework with a lot of SaaS-specific features built in. With each new project I implemented more and more features on it, to the point where I can now launch a functional MVP quite rapidly.
Lately (like, this week!) I got the idea of trying to license this framework to others, as well as sell them my web development services for the rest of their project. To this end, I published a quick LP (http://turbomvp.com/), which I'm now trying to improve, because I've been told it's not really clear at all what I'm offering :)
I haven't yet figured out the licensing model either, this is still a very rough idea. The framework, however, is really cool.