I was wondering the best priced PREPAID mobile broadband (pref. unlimited access). I live in Europe BUT I will need it when I go to the US (Florida, and Philly) so can some recommend a good company to deal with please?

I know I will need a dongle of some kind so if the company won't mail one to me over here I can buy one on ebay (I took a peek and saw they have LOTS to choice from).