multisite - plugin to allow users to create their own site?

In your blog at:

She mentions:
"You can create sites manually or you can install a plugin that lets users create their own. I’ll cover how you do this with a plugin later, but first, here’s how to create a site manually."

But I don't see any references to that. Is there a plugin (yours, or somebody else's), that will allow an individual to sign up, and automatically generate a new site (and login) that I can use?