The IRS today released 2006 IRS Data Book (Publication 55B).  From IR-2007-63:

The report describes activities conducted by the IRS from October 1, 2005, through September 30, 2006, and includes information about returns filed and taxes collected, enforcement, taxpayer assistance and the IRS budget and workforce.

During Fiscal Year (FY) 2006, the IRS collected more than $2.2 trillion in tax and processed over 228 million returns. Over 80 million returns, including 54.3% of individual income tax returns, were filed electronically in FY 2006. Over 108 million individual income tax return filers received tax refunds totaling $243 billion. In FY 2006, IRS spent an average of 42 cents to collect each $100 of tax revenue.

IRS examined nearly 1.3 million individual income tax returns in FY 2006, more than double the number examined in FY 2000. Examinations of business tax returns grew for the second year in a row, reaching over 52,000 in 2006. IRS personnel answered over 32.6 million toll-free calls from taxpayers, and the IRS Web site received about 194 million visits.