SIM with 3GB of Data for $5 from Dodo Mobile Broadband
By comparison, Telstra's 3GB mobile broadband topups cost $50 while Optus charge $40 and Vodafone $39 - all with a 30 day expiry. It says on the LivingSocial offer that the limit is "1 voucher per user", although it would be interesting to know whether that limit is on LivingSocial's side (in which case 1 user = 1 registered email address) or on Dodo's side (where 1 user = 1 registered physical address).
This offer includes a SIM card suitable for both standard and micro slots, 3G/HSDPA speeds, you won't get locked into any contract, and delivery of the SIM is free.
You will have 30 days in which to activate the SIM, which begins as soon as your card is dispatched. Apparently is can take up to 3 weeks to be delivered though, so you may have to use the SIM fairly quickly after receiving it.