Use Cheetah (formerly Intelliconnect, formerly CCH Internet Tax Research Network). 

For historical tax information, check out Taxation and Economic Reform in America, A Historical Archive 1781-2009.

Search the IRS Tax Code, Regulations, and Official Guidance, as well as Tax Stats.

Use the IRS's Audit Technique Guides.

Go to IRS Tax Forms and Instructions.

Look at the Tax Policy Center.