This came up in the discussion of the Nutch Tutorial and I thought it might be helpful to have an entry on the site.
It is a collection of hand-edited resources which as of today claims:
4,952,266 sites – 92,824 editors – over 1,008,717 categories
The information you will find under the “help” menu item will be very valuable as you learn to make sure of the data files from this source.