Buildout is a "system for managing development buildouts", according to its home page. While often identified as a Zope project, and indeed licensed under the ZPL by Zope creator Jim Fulton, buildout is useful for configurations beyond Zope, and even, in rare cases, a few that have nothing to do with Python. The home page above includes extensive documentation, examples, and more.
At one level, buildout (still referred to as "zc.buildout" on occasion) manages eggs.
[explain relation to easy_install and eggs; workingenv.py and virtualenv.py; explain usage]
Other treatments of buildout include:
Martin Aspelli's book on Plone 3, which includes a good reference on buildout;
the Zope 3 Wiki includes a useful page on the subject;
Jim wrote a tutorial;
A proposal is being floated to create a more professional site for buildout. At a first step, let's collect notes on what such a site would contain.