This wiki collects information about prepaid (or PAYG) mobile phone plans from all over the world. Not just any plans though, they must include good data rates, perfect for smartphone travellers, as well as tablet or mobile modem users.
Adafruit sells working 2G SIMcards for use in telephony enabled projects. hacking hardware The manufacturer/provider is Ting. GSM Pay as you go provider, kind of like voip.ms.