travel insurance reviews usa

Travel insurance is mandatory for travel to the U.S.?
My parents are traveling to U.S. in February 2008. I just want to know if the travel insurance is mandatory for international travelers coming to the U.S. family visits? If so, can I have some plans insurance companies of recognized prestige (at lower cost), they have good customer feedback? I have heard that "Travelers Freedom" plan is very cheap and good for the short trip, but do not know about the reputation and efficiency claims of the company offering the plan.
Exactally not mandatory but I think there is a requirement that the visitor has sufficient resources to be financially responsible. The best place to find travel insurance is a travel insurance comparison site as http://www.quotewright.com
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