At a bare minimum, you'll need the following (for both the buildmaster and a buildslave):
Buildbot requires python-2.3 or later, and is primarily developed against python-2.4. It is also tested against python-2.5 .
Both the buildmaster and the buildslaves require Twisted-2.0.x or later. It has been tested against all releases of Twisted up to Twisted-2.5.0 (the most recent as of this writing). As always, the most recent version is recommended.
Twisted is delivered as a collection of subpackages. You'll need at least "Twisted" (the core package), and you'll also want TwistedMail, TwistedWeb, and TwistedWords (for sending email, serving a web status page, and delivering build status via IRC, respectively). You might also want TwistedConch (for the encrypted Manhole debug port). Note that Twisted requires ZopeInterface to be installed as well.
Certain other packages may be useful on the system running the buildmaster:
If your buildmaster uses FreshCVSSource to receive change notification from a cvstoys daemon, it will require CVSToys be installed (tested with CVSToys-1.0.10). If the it doesn't use that source (i.e. if you only use a mail-parsing change source, or the SVN notification script), you will not need CVSToys.
And of course, your project's build process will impose additional requirements on the buildslaves. These hosts must have all the tools necessary to compile and test your project's source code.