A new generation passthrough card. If one does not have baseband 01.59.00 on their iPhone 4 (GSM model), this is one of the only ways to unlock the iPhone for unauthorized carriers.
The Gevey SIM is a "tethered" unlock, requiring an emergency call to 112 in order to connect after every baseband reboot. Its legality has been questioned multiple times. With FuriousMod (see link below) it is not necessary to manually dial emergency numbers, so the chance of error is smaller.
Gevey Ultra does not work with a baseband of an iOS 5.x version (04.11.04 or higher); it is only compatible with basebands 04.10.01 and lower.
Gevey Ultra S was announced to be able to unlock iPhone 4S (GSM) running iOS 5.0 and 5.0.1. Specific basebands it is compatible with are 01.0.11, 01.0.13, and 01.0.14.