Ultrasn0w!

Blacksn0w unlocks your jailbroken iPhone, allowing it to be used with any GSM carrier (AT&T and TMobile in the U.S.) without a contract. It can be installed through blackra1n, the jailbreaking application by geohot.

redsn0w is a utility that will jailbreak your iPhone or iPod Touch, allowing it to run apps that are not approved by Apple.

BootNeuter will unlock your jailbroken iPhone 2G (running iOS 2.x or lower). It is available in the Cydia app store.

PwnageTool is an advanced jailbreaking application by the iPhone Dev Team for Mac OS X. It can build a pre-jailbroken .ipsw ("iPhone software") file with apps and anything else pre-loaded, so that you can run apps without the approval of Apple. PwnageTool can also hacktivate your phone, and lets you choose a custom boot logo.

sn0wbreeze is an advanced jailbreaking tool that lets you build a custom IPSW ("iPhone software") file. It allows you to jailbreak your iPod or iPhone, so that it can run apps unapproved by Apple. sn0wbreeze can also set up a custom boot logo, hacktivate your iPhone (if necessary), root partition size, and more.