UnitedStatesVisas Website
An official source of information about U.S. visa policy and procedures.
http://www.unitedstatesvisas.gov/

Path2usa
Path2USA.com strives to answer your every query about any thing that's relevant to the United States. From immigration information to adjusting to American lifestyle, phone cards to visitor's insurance, classifieds to matrimonials, we cover the whole gambit. The goal is to provide answers to your questions in a clear and concise manner, without you having to search through oodles of sites on the web. They do that for you.
http://www.path2usa.com/

Visitors Coverage
VisitorsCoverage is a unique one-stop resource designed to meet international visitors' medical insurance needs. They offer online quotes that can be compared against one another, alternative plans and low-cost solutions that would fit any budget.
http://www.VisitorsCoverage.com

More Visitor Insurance Websites
Best Visitor Insurance
Visitor Visa Insurance
Inbound USA Insurance
USA Visitor Insurance

Bureau of Consular Affairs, U.S. Department of State
The Executive Branch and Congress have constitutional responsibilities for U.S. foreign policy, and within the Executive Branch, the Department of State is the lead U.S. foreign affairs agency. The Department of State plays the primary role in leading interagency coordination in developing and implementing foreign policy; manages foreign affairs resources; leads and coordinates U.S. representation abroad; conducts negotiations and concludes agreements and treaties on issues ranging from trade to nuclear weapons; and coordinates and supports international activities of other U.S. agencies and officials.
http://travel.state.gov/

Deptartment of Homeland Security 
The Department of Homeland Security through its components, programs and initiatives is responsible for maintaining the security of U.S. borders, providing immigration services, and welcoming visitors.
http://www.dhs.gov

United States Customs and Border Protection
U.S. Customs and Border Protection’s top priority is to keep terrorists and their weapons from entering the United States. While welcoming all legitimate travelers and trade, CBP officers and agents enforce all applicable U.S. laws. CBP prevents narcotics, agricultural pests and smuggled goods from entering the country and also identifies and arrests those with outstanding criminal warrants. On a typical day last year, CBP officers welcomed 1.2 million people at the nation’s 314 land, air and seaports while denying entry to more than 3,000 inadmissible visitors.
http://cbp.gov/

United States Citizenship and Immigration Services
The USCIS is responsible for the administration of immigration and naturalization adjudication functions and establishing immigration services policies and priorities.

These functions include:
adjudication of immigrant visa petitions;
adjudication of naturalization petitions;
adjudication of asylum and refugee applications;
adjudications performed at the service centers, and
all other adjudications performed by the INS.

http://www.uscis.gov