These are instructions on how to jailbreak your iPod Touch 3G and iPod Touch 4G using Greenpois0n for Windows. Greenpois0n supports iOS 4.1 and iOS 4.2.1.
Instructions on how to jailbreak and unlock your iPhone 3G on the iOS 4.2.1 firmware using RedSn0w for Mac. This tutorial can update your baseband if needed for unlock.
The iPhone Dev-Team has announced RedSn0w 0.9.15b1 which brings numerous new features including the ability to restore to older firmware not being signed by Apple.
Saurik has released an iOS 8 compatible version of Cydia for manual download following the release of an untethered Pangu jailbreak for iOS 8 - iOS 8.1.