A year ago (or at least over Christmas and New Years) I was playing a lot with Django (and reading about WSGI) so its fitting I ran across Aspen.
Aspen is designed around the idea that there are basically two kinds of websites, publications and applications, differentiated by their organization and interface models. A publication website organizes information into individual pages within a hierarchical folder structure that one navigates by browsing. In an application website, on the other hand, data is not organized into hierarchical pages but is dealt with via a non-browsing interface such as a search box.
The HTML version of this documentation is an example of a publication website: a number of hypertext documents organized into sections. If we weren't using LaTeX (or if I knew how to use it better), the sections would probably be encoded in folders. Gmail is a pure application website, one which organizes and presents information non-hierarchically. Most websites, however, are hybrids. That is, within an overall hierarchical organization you will find both individual pages of information as well as applications such as a site search feature, or a threaded discussion forum.
Publication websites are actually a subset of application websites, of course. An application site can use any interface metaphor; a publication is an application that uses the familiar folder/page metaphor to organize and present its information. Therefore, every website is fundamentally an application.
Aspen enables the full range of websites: publications, applications, and hybrids. It uses the filesystem for the hierarchical structure of publication and hybrid websites, and provides a mechanism for including applications within that hierarchy.
Based on the screencast, it looks very cool. Why? It is so un-Ruby: well documented, it supports multiple frameworks (a Python HTTP server that support PHP!) and Conan O'Brien-style talking faces. Hopefully I'll be able to squeeze some time away from baby care to play around with it.