For those in the UK, the available Government datasets are published on http://www.data.gov.uk

The datasets are not public domain, but licensed under the Open Government Licence (which allows you to use and adapt the data for commercial use).

There's also the Global Open Data Index: a website that ranks countries by how much Government data is available as open datasets based on certain criteria. The current top spot is taken by Taiwan

  1. Taiwan
  2. UK
  3. Denmark
  4. Colombia
  5. Finland
  5. Australia
  7. Uruguay
  8. USA
  8. Netherlands
  10. Norway
  10. France

http://index.okfn.org/place/