Federal Tax Code — Full text of the U.S. Tax Code. Searchable.
US Code: 26 U.S.C. - Internal Revenue Code — Internal Revenue Code.

Applicable Federal Rates — Federal tax rates for short and long term instruments.
Income Tax Treaties — Information on adverse Tax Court rulings. From 1997.
IRS Information Letters — Advisory letters that interpret principles of tax law without applying them to a specific set of facts. From 2000.
IRS Written Determinations — Private Letter Rulings, Written Determinations comprise Technical Advice Memoranda (TAM) and Chief Counsel Advice (CCA). From 1999.
Tax Court Rules of Practice and Procedure — Full text rules.
Tax Info for Business — Small and midsize business tax assistance from the IRS.
United States Court of Federal Claims — Decisions since 1997. Jurisdiction includes government contracts, income tax refunds, Native American claims and more.