Virgin Mifi
The Virgin Mobile Mifi offers more-or-less unlimited (slower after 5 GB) for $40 a month, no contract. It uses the Sprint network, so same technology as Verizon. You can also get 10 days/ 200 MB if you're not using it much.
It works pretty well, but as everything it depends where you are. Dependability in the Atlanta area is only middling. It's nice if you want to be able to connect more than one device at a time, such as two people each using a laptop or ipad.
I don't know any good solution to the people who are outside ATT's service areas and want to connect in Europe. There exists a GSM Mifi, but it's expensive, and I was told by ATT that ATT wouldn't sell me a sim card for it. Hopefully, these systems will change and improve in the next year or two.