LimeRa1n is for 4.0.2 and higher on all iDevices.
GreenPois0n is for 4.1 on all iDevices
PwnageTool is for 4.1 also on all iDevices
Redsn0w is for 4.1 and all previous iOS version
PwnageTool and Redsn0w have been around for a while. Both created by the iPhone Dev Team (most trusted group of hackers).
GreenPois0n was created by the Chronic Dev Team
LimeRa1n is GeoHotz jailbreak. Creator of BlackRa1n and PurpleRa1n.
In my opinion I like PwnageTool the best because you can upgrade to the newest iOS while preserving your baseband. On the other jailbreaks you can not do that.