This is a tough one: while Entransit already has at least a couple of what look like home pages (see below), it deserves an explanation better suited to the context of the Python Wiki. Its ecological niche is a challenge to explain, though ...
Entransit is open-source software (exclusively? predominantly? This answer might change in 2007-2008). It was originally created to effect "content delivery" as a front end for Plone as a back-end "content management system". This is a WebFrameworks piece for people with big sites. Notice that, among other advantages, this facilitates separation of servers, so that content delivery can be managed with an eye to security, scalability, and availability, while content management focuses on flexibility and functionality.
Entransit incorporates search technologies, or at least "hooks", and explicitly invites front-end development with Java, .NET, and PHP.
The Plone site summarizes, "The Entransit Content Delivery System allows a hybrid approach to content management that creates an open source alternative to expensive mid-tier content management systems. . . ."