IRS Tax Forms and Instructions - You can download almost any IRS form with instructions from this site.

SS-4 Online - Now you can fill out an SS-4 directly online when starting a new business!

Tax Regulations - List of tax regulations issued since August 1, 1995, with references to plain language summaries where available.

U.S. Tax Code - The United States Code contains the text of current public laws enacted by Congress. The U.S. Code does not include regulations issued by executive branch agencies, decisions of the federal courts, or treaties.

IRS Site Map - Looking for an IRS site not listed here?  Slice through the confusion and go directly to their site map.

Where To File Federal Tax Returns - Mailing addresses for your federal tax returns.

IRS Withholding Calculator - A handy tool to determine how much you should withhold from your paycheck.

Smbiz.com - An excellent site with a tremendous amount of useful tax information:

Administrative

Ordering Employer Tax Forms
IRS Phone Numbers
Electronic Payment Options
Employer ID Number Fax Request
State Tax Telephone Numbers

Depreciation

Federal Depreciation Rates  

Automobile Rates

Depreciation Adjustment On Mileage Rate
Cents Per Mile Valuation Rule
Luxury Car Depreciation Limits
Annual Lease Value Tables
Lease Value Inclusion Amounts

Interest Rates

IRS Rates On Under/Over Payments
Applicable Federal Rates
Blended Annual Applicable Federal Rates

Fringe Benefits

Per Diem Amounts - High/Low Rates
Standard Industry Fare Level
W-2 Life Insurance Add-Back Table

Business

Glossary of Tax and Business Terms (A to M)
Glossary of Tax and Business Terms (N to Z)
Business Abbreviations

Internal Revenue Bulletins - Keep up with the newest changes in procedure and official rulings! This section of the IRS site brings you Treasury Decisions, Executive Orders, Tax Conventions, legislation, court decisions, and other interesting information in a series of weekly publications.

IRS Publications - The IRS publishes guides on hundreds of different topics.  You can download a specific IRS Publication at this site.

IRS Comment on Proposed Regulations - An IRS site which lists the proposed regulations that are available for public comment.

Taxpayer Advocate - If you are having a problem with IRS compliance (either filing or paying), you can try their taxpayer advocate service.

IRS Tax Topics   An excellent IRS site with topics organized by category

IRS Frequently Asked Questions  The most frequently asked questions posed by category and sub-category

IRS Information For Business  Contains a summary of information useful for new and existing businesses.

Valuation Guide For Noncash Charitable Contributions - There is no definitive guide for valuing your old underwear for charitable purposes, but this site will give you some idea of what the average thrift shop values are.