There are in fact only a few comparison sites in the U.S., but dozens of lead generation sites where you enter your details and then get calls from agents (often asking for the same details again).
These sites are nearly worthless, as they only give you quotes from a few selected companies (usually the companies willing to pay for leads). None of them includes more than 2 or 3 of the top 5 carriers by price and customer satisfaction, based on J.D. Power surveys.
That's why I built EZquo (http://ezquo.com). It's a simple website that lets you enter your details only once and get a private link that you can pass to any agent for a quote, saving you hours as well as your sanity.