Stefan Esser, better known as i0n1c, is a prominent member of the jailbreak community. His untethered exploit was responsible for the jailbreak that lasted from iOS 4 to 4.3.3, and he has been contributing to the Dev Team for quite some time.
i0n1c's untethered jailbreak may have been killed in iOS 5, but he's got a new and very controversial idea for another iOS 5 jailbreak in the future: blocking access to pirated apps.