Datasets which are publicly available (or available upon request) and contain location-data and/or vary over a period of time.
ftp://ftp.ncdc.noaa.gov/pub/data/
Not sure that is the sort of data you want. In the U.S., lots of states and the federal government also have extensive geographical data sets (so you mind find one with a theme you are more interested in).
Census data
Maybe wikipedia's data is available?
Is there some open search engine, which gives its data free?
There could be more but I'm tired and this will hopefully be of help.
