It's a great new open source tool that helps you support all versions of python with your existing automated tests. It's not a test runner, it's a super test runner runner! Or something. There is no easy way to describe it which is why the front page of the website is sort of confusing, IMO. We use tox on the Nose project to make sure each code change doesn't break the tests in Python 2.4, 2.5, 2.6, 2.7, 3.0 or Jython. Not everybody targets so many environments but if you want to maintain a widely used module then you probably will want to use tox someday.
Here's our Hudson matrix setup w/ tox: http://hudson.testrun.org/job/nose- unstable/ (we just set this up recently which is why most builds were broken in one or two envs) I can also spend some time going over how we are currently maintaining a test suite that runs in both 2.x and 3.x simultaneously.
This will be our Best Meeting Ever!