Are you specifically looking for prepaid WiFi service? You can following the link to find a discussion about prepaid WiFi and prepaid Wi-Fi plans. Most, if not all newer computers, specifically laptops and notebook computers, have Wi-Fi built-in, so there would be no need for you to have a specific prepaid USB WiFi modem. In other words, so long as you can connect to a WiFi signal using any means you have available to you, you will be able to take advantage of either free WiFi service, or purchase prepaid WiFi plans.
If your computer doesn't have WiFi built-in, you can purchase a WiFi card or USB WiFi. If this is in fact what you're looking for, please let me know!
If you're looking for something more mobile (i.e. giving you more broad coverage while you're on the go), you can also check out the prepaid broadband plans discussion. In the comparison chart you'll find a row that notes which operating system(s) are compatible with each of the modems, which will help you find the ones that will work with MACs. Prepaid mobile broadband is becoming more and more affordable, and I'm expecting some big pricing moves this year!
