I found a couple of nice and comprehensive ones. If you have any, just share it here.
  1. UCI repositories: http://archive.ics.uci.edu/ml/
  2. Infochimps: http://www.infochimps.com
  3. New York university: http://pages.stern.nyu.edu/~adamodar/pc/datasets/
  4. The Office of National Statistics in the UK:  http://www.statistics.gov.uk/hub/people-places/
  5. Machine learning dataset repository: http://mldata.org
  6. Computer vision/Image processing: http://www.imageprocessingplace.com/root_files_V3/image_databases.htm