Here is a list of some of my favorite resources
http://www.ofheo.gov/media/hpi/4q08hpi.pdf
http://www.biggerpockets.com/dashboard
http://www.zilpy.com
http://www.realtor.com/
http://www.loopnet.com/
http://www.census.gov/
http://activerain.com/
http://www.realtraininginstitute.com/
What online resources do you use on a regular basis?