FreedomPop is offering a free trial of their family plan. It features unlimited talk, text and 4GB of shared data for just the cost of a SIM card. SIM cards are currently being priced at 50¢ each so you can get two free lines of service for one month and 2 SIM cards for just a dollar.
FreedomPop's family plan also includes unlimited MMS and visual voicemail, and it operates on AT&T's network. After the free trial is up, subscribers can expect to pay $44.99/month for the shared data plus a $5.00 per line monthly access charge or a total of $54.99 for two lines. This is currently the cheapest offering around that I'm aware to offer such a feature set using AT&T's network.
To take advantage of this free trial offer, head on over to the FreedomPop website.