Virgin Mobile was originally started with the idea of selling only prepay phones requiring no contracts whatsoever. While initially only a couple of daily options were available, there are now some monthly plans available as well. And true to the company's original intent, there remains no long term contract requirement.
Unlike Tracphone which requires a minimum of US$20 every two months, the recharge rate for the Virgin daily plans requires a minimum recharge of US$20 every three months. Which is great if you only use a cell phone for emergency type situations, or fixing scheduling screwups after you have left the nearest landline.