You have two choices in how to get Nutch:
You can download a release here. At the moment the latest release is 1.6.
Or, you can check out the latest source code from subversion and build it with Ant, this gets you closer to the bleeding edge of development. The trunk (from which nightly builds are built) is under heavy development with bugs showing up and getting squashed fairly frequently.
There is a separate repository for Nutch version 2 development and some documentation can be found on the FrontPage but please discuss this with the developers before looking for it