It's a templating engine. You write your documents in an outliner like JOE or Radio UserLand and it turns them into web pages for you.
First, you make a template file that defines what you want your web pages to look like. What the title should look like, what a heading should look like, and so on.
Then, you write your web pages in an outliner. Outline depth (indentation) determines what style should be applied to your pages. For example, this page looks like this in JOE:
Once you've finished writing, you run Upstreamer, which finds which files you've changed, calls the TabTemplate renderer to make the HTML, then sends them (via FTP) to your website.
Easy!
Not yet!
This is part of a larger project. If I don't finish that, it might be free ... but you can't have it yet, sorry :)
For Frontier:
For Radio: