FIRSTGOV - U.S. Government's Offical Web Portal. The federal government's initiative to index all government web sites and provide comprehensive searches...
(START HERE! if you mainly want to browse)

GOOGLE Uncle Sam - One of the best keyword search engines for searching government information on the internet, but no menus
(START HERE! if you know specifically what you are looking for)

GOVSPOT - Easy to use menu-driven entry point for miscellaneous Federal, State, & Local government information
(Good for browsing & does have keyword searching)

GPO Access - Principal entry point for U.S. Government core publications via the internet plus much more. The Government Printing Office's "GPO Access" provides access to such publications as the the Code of Federal Regulations, the U.S. Code, congressional bills, slip opinions of the Supreme Court, the Catalog of Government Publications,etc.
(Note: see their "A-Z Resource List" Link)

SCIENCE.GOV - Science Information from the U.S. Government