Hypothesis is a library for testing your Python code against a much larger range of examples than you would ever want to write by hand. It's based on the Haskell library, Quickcheck, and is designed to integrate seamlessly into your existing Python unit testing work flow.
Hypothesis is both extremely practical and also advances the state of the art of unit testing by some way. It's easy to use, stable, and extremely powerful. If you're not using Hypothesis to test your project then you're missing out.
Hypothesis works with most widely used versions of Python. Officially it supports 2.6, 2.7 and 3.3+, but 3.0 through 3.2 will probably also work in the unlikely event that you're using them. Both CPython and PyPy are fully supported.
To learn more about how to use Hypothesis, extensive documentation and examples of usage are available at readthedocs.
If you want to talk to people about using Hypothesis, we have both an IRC channel and a mailing list.
If you want to contribute to Hypothesis, instructions are here.
If you're looking for inspiration for writing your own QuickCheck clone, here's a list of ideas you might want to borrow.
If you want to hear from people who are already using Hypothesis, some of them have written about it.