SOI Tax Stats : Corporation Tax Statistics
URL: http://www.irs.gov/taxstats/bustaxstats/article/0,,id=97145,00.html

Tax Statistics : Produced by the Statistics of Income Division and Other Areas of the Internal Revenue Service
URL:   http://www.irs.gov/taxstats

IRS Data Book
URL:   http://www.irs.gov/taxstats/article/0,,id=102174,00.html