Other sources of data sets
R packages -- type "data()"
gene sequence data sets at NIH -- ncbi.nlm.nih.gov/genbank
protein data (BLAST) -- blast.ncbi.nlm.nih.gov/Blast.cgi
social network data -- snap.stanford.edu/data
FRED (St. Louis Federal Reserve) -- http://research.stlouisfed.org/fred2
Quandl provides 2 million financial and economic time-series datasets from over 250 sources -- http://www.quandl.com/
Thinknum - a new free data service similar to Quandl.com in that they organize and list publicly available data sets.
Research quality datasets gathered together by Bit.ly's Hilary Mason -- https://bitly.com/bundles/hmason/1
Citizen Statistician blog -- http://citizen-statistician.org/2012/11/07/data-sets-a-list-in-flux/
Data Hub contains over 5000 data sets -- http://datahub.io/
Electricity Data Browser by the U.S. Energy Information Administration -- http://www.eia.gov/electricity/data/browser