Choose the right software to jailbreak an iPhone
You now have several choices on how to jailbreak an iPhone. The question now is whether you want to upgrade you iOS to the latest version, and for whatever reason retain an older iOS version. Depending on the iOS version as well as your iPhone model, there are several ways to jailbreak an iPhone.Jailbreak an iPhone if you don't need to unlock
As posted in other pages on this site, if you jailbreak an iPhone, you need to first find out if you have an unlock available for the baseband that you will have to upgrade to. If you depend on a carrier unlock, avoid upgrading your iOS unless you're sure there is an unlock for it as well. The following ways to jailbreak an iPhone almost invariably requires an upgrade of the iOS, and for Redsn0w for example, there is no unlock for the 4.3.3 stock upgrade. If ever, you will need a custom iPSW to upgrade your iOS to avoid modifying your baseband to an unlockable level.Most popular software used to jailbreak an iPhone
Redsn0w 0.9.6rc16, designed for the iPhone on 4.3.3. This is an untethered Redsn0w jailbreak, and has solved the side switch vibration and battery drain problems that bothered iPhone 3GS and 4 users with the earlier release. The Redsn0w jailbreak download works for iPad 1, iPhone 3GS, iPhone 4 and the Apple TV 2G on 4.3 version 8F202. For those with Apple TV2G 4.3 version 8F305, you'll need an attachment included in your PwnageTool app folder to properly port this Redsn0w jailbreak download version. Redsn0w 0.9.6rc16 can be downloaded for Mac and Windows to jailbreak an iPhone.
For instructions on how to unlock an iPhone using Redsn0w, click this link.
GreenPois0n Jailbreak for iOS 4.2.1. Some people prefer to keep their iOS to 4.2.1, which is a great fit for GreenPoison by the Chronic Dev Team. There is a jailbreak download available for those on iOS 4.2.1. It's an untethered jailbreak download and allows you to "untether" your iPhone, iPad and even the Apple TV 2G. You can get your jailbreak download for the GreenP0ison jailbreak for iOS 4.2.1 for Windows and Mac. You can check BestJailbreakSoftware for a few shortcuts.
Another popular way to jailbreak an iPhone is by using the PwnageTool 4.3.1 untethered exploit. However, developers recommend Redsn0w for most users as it is easier to implement. Besides, PwnageTool 4.3.1 runs only on a Mac computer, so Windows users may be out of luck. For instructions on how to use the PwnageTool 4.3.1, check out this site, and you'll be able to download the custom IPSW and jailbreak here as well.
No one jailbreak software works for all versions of the iPhone. Doing your research and reading the forums will help you flawlessly jailbreak an iPhone.