greenpois0n is described as 'Greenpois0n performs an untethered jailbreak for iOS 4.2 on the iPhone 4/3GS, iPod touch 2G/3G/4G, iPad, and Apple TV 2, allowing them to run apps unapproved by Apple. It uses the limera1n exploit by geohot' and is an app in the System & Hardware category. There are three alternatives to greenpois0n for iPad, iPhone, Mac and Linux. The best alternative is checkra1n, which is free. Other great apps like greenpois0n are seas0npass (Free, Open Source) and unc0ver (Free).
seas0npass offers an easy way to jailbreak your Apple TV (2nd generation) running iOS 4.2. It downloads and jailbreaks a .ipsw file (iPhone software), which is then restored to your Apple TV through iTunes.
unc0ver is a jailbreak, which means that you can have the freedom to do whatever you would like to do to your iOS device. Allowing you to change what you want and operate within your purview, unc0ver unlocks the true power of your iDevice.