Internal Revenue Service: Digital Daily (http://www.irs.ustreas.gov/)
This is the most useful site for finding tax forms and publications, and includes resources for individuals, businesses, charities/non-profits, and tax professionals. An exhaustive list of frequently asked tax-related questions (organized by category and keyword) may be found here .
FedWorld Tax Form Search (http://www.fedworld.gov/taxsear.html)
Site allows users to locate archived IRS Forms and Publications. This site is easy to use, but not often updated.
Tax Statistics (http://www.irs.ustreas.gov/taxstats/index.html)
Provides links to data files compiled from samples of tax and information returns filed with the IRS, the IRS statistical overview, statistics by topic (individuals, corporations, or tax exempt organizations), statistics by form, and links to the IRS Data Book, the Statistics of Income Bulletins, and more.
U.S. Tax Code Online (http://www.fourmilab.to/uscode/26usc/ustax.html)
This document allows users to access the complete text of the United States Internal Revenue Code (Title 26 of the U.S. Code (26 USC)) in a variety of ways. Hyperlinks embedded in the Code permit follow cross-referencing between sections with a simple mouse click. A WAIS-based full-text search engine allows you to quickly locate Code sections by content.