How long will you need it for
Are you looking for it for a laptop or some other device that is not usb?
There is only one company currently offering unlimited access that I am aware of, Virgin.
Virgin mobile probably has the best prices currently, you can get unlimited 3g for $40 a month with their mifi, though you need to buy the mifi (currently $130) or a usb stick for $80. If you only need it for ten days, they will do 100 megs for $10 for ten days.
Tmobile has three plans, $10 a week for 100 megs, $30 a month for 300 megs, or $50 for 1 gb.
AT&T offers the same three plans as t-mobile, but at $15, $30 and $50
Verizon same as AT&T, but they also offer an $80 one month plan that goes to 5 GB