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 Web framework piece for people with big sites.
Entransit incorporates search technologies, or at least "hooks", and explicitly invites front-end development with Java, .NET, and PHP.
The [http://plone.org/products/entransit 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. . . ."
["Enfold Systems"] hosts [http://www.enfoldsystems.com/Products/Entransit the authoritative home page for Entransit] as one of its product offerings.