Does Python have a unit testing framework compatible with the standard xUnit style of test framework? If so, what is it, where is it, and is it any good?
nose seems to be the best combination of flexibility and convenience.  It runs unittests, doctests, coverage (with an extension) and py.test-like tests from one framework and does so admirably.  It has enough popularity that it has had some IDE integration done as well for Komodo Edit and I wouldn't be surprised to see it elsewhere as well.
I like it for one strong reason: I almost always doctest before writing more extensive tests in another framework.  This is because, for basic tests, doctests kill two birds with one stone.  You get executable tests (although they are a bit clumsy to write well sometimes) as well as API documentation and interactive documentation at the same time.  nose will run these with the bundled doctest extension when you use a command-line option (--with-doctest).
I say this having come from py.test as my former favorite.  While it is great, nose tests are similar enough to me that I don't miss it, and I like the integration of the various test methodologies under one roof, so to speak.  YMMV, but I recommend taking a good look at nose before choosing another.  If you aren't familiar with py.test tests, you should look at them as well.  I find them terrific because they are usually written in such a way that they can be easily debugged without the testing framework, which makes one less tricky system involved in the debugging session.  I find that alone invaluable, while they are also easier to write than unittest tests in my opinion.
Unit tests in Python