The Internal Revenue Service
Downloadable tax forms and publications as well as a lot of great tax advice.

Hiring Incentives to Restore Employment (HIRE) Act
To be completed by new employees.

Social Security Administration
Learn how the Social Security Administration can help with your retirement, medicare, disability, survivorship and other benefit questions.

U.S. Small Business Administration
You don't need an MBA to use the SBA. This site outlines everything in plain English. The SBA site contains a wealth of information for entrepreneurs and small business owners.