Karma is perhaps the most unique MVNO out there. Their only product is $99 personal hotspot, and you can then buy data (which never expires) for $14/1GB, or $99/10GB - with no contracts, monthly fees, or fine print.
The Karma hotspot is designed to be shared - and if it is turned on, it is open to other users. If another user connects through your Karma hotspot you both earn a bonus 100MB of data, and their usage does not count against your data pool.
The downside of Karma is that the current Karma hotspot is tied to Sprint’s soon to be eliminated WiMAX network. But later this year Karma will be moving to Sprint’s LTE network, and they promise a cheap and easy upgrade path for current Karma users.
They also intend to eventually offer international service too - and wherever you go the data you have purchased stays with you and never expires.
Karma's vision is to offer wireless data as cheaply as possible to as many people as possible. Since the data never expires - once the version on Sprint's LTE network launches I think Karma will make for a great backup data connection.