United States Code
    U.S. House of Reps. site
    Cornell Law School

Code of Federal Regulations
    Government Printing Office
    Parallel Table of Authorites

Supreme Court Cases (decided cases)
    Supreme Court site
    Findlaw.com (may require free membership to look up old cases)

Lower Court Cases
    Findlaw.com (scroll down to "Case Law")

Tax Court Cases & Memos
    U.S. Tax Court
    Introduction to tax court
    Tax Court Memo & Opinion Search
    Docket Inquiry
    Judges

Congressional tax committees
    U.S. House Ways and Means Committee - House committee that writes the tax laws.
    U.S. Senate Finance Committee - Senate committe that writes the tax laws.

Infernal Revenue Service
    Internal Revenue Manual
    Delegation Orders

Rules and Procedures
    Federal Rules of Criminal Procedure
    Federal Rules of Civil Procedure
    Tax Court Rules & Forms