Wikipedia
JailbreakMe is a series of jailbreaks for Apple's iOS mobile operating system that take advantage of flaws in the Safari browser on the device, providing an immediate one-step jailbreak unlike more common jailbreaks, such as Blackra1n and redsn0w, that require plugging the device into a computer and running the jailbreaking software from the desktop. Jailbreaking allows users to install software that is not approved by Apple on their iPod Touch, iPhone, and iPad; JailbreakMe automatically includes Cydia, a package management interface that serves as an alternative to the App Store.
JailbreakMe's first version in 2007 worked on iPhone and iPod Touch firmware 1.1.1, the second version was released in August 2010 for firmware 4.0.1 and earlier, and the third version was released in July 2011 for iOS versions 4.3 to 4.3.3 (and was the first jailbreak for the iPad 2). JailbreakMe 3.0 has been used to jailbreak at least two million devices.