Less than a decade ago, MVNOs were the hot new kids on the block that everyone wanted to befriend. But a few basically dried up and blew away (Amp'd Mobile and Disney Mobile), while others morphed into carrier sub-brands (Boost Mobile and Virgin Mobile USA). Some, such as Tracfone and Kajeet, have managed to keep plugging away with niche offerings or cut-rate pricing. But a new crop of MVNOs is emerging, some with offerings--such as family data plans--that the major operators would prefer to avoid. See what the future holds in this quick primer on notable MVNOs.