Here are some great tax research sites for taxpayer and practitioner alike.

INTERNAL REVENUE SERVICE
United States Federal income, gift and estate taxation.

STATE TAX AUTHORITIES
Links to all fifty U.S. states.

TAX WORLD
University of Illinois's Professor Omer provides this site for tax information for internet users.

ESSENTIAL LINKS TO TAXES
The definitive tax link's page

REVENUE CANADA
A comprehensive page of Canadian Federal tax resources.  This is the english site-they're also on the web in French.

INLAND REVENUE
United Kingdom income taxation, including their new self-assessment tax.

TAX AND ACCOUNTING SITES DIRECTORY
Dennis Schmitt, associate professor at University of Northern Iowa, provides the last word on tax links pages