Interesting data on 1996-2010:
http://www.census.gov/hhes/www/socdemo/voting/index.htmlIn 2010, of the ~211 million citizens aged 18 and above, ~137 million of them were registered to vote and ~96 million of them voted in the November 2010 elections:
http://www.census.gov/hhes/www/socdemo/voting/publications/p20/2010/Table4a_2010.xlsThere's also some interesting state maps on voter turnout (as a % of eligible, not % of registered) where you can compare the drop in turnout between presidential and mid-term elections: