iPhone 4 was recently jailbroken and now it is unlocked too, thanks to the Dev Team. It cannot be easier, no PC/Mac required, only the internets!
All you need are the following :

A Jailbroken iPhone 4 / 3GS,3G (it should be activated, that is a pre-req for online jailbreak)
A WiFi connection
That is pretty much it!

Step by step (best practices) :

Launch Cydia and search for ultrasn0w [if not found, then add repo : repo666.ultrasn0w.com in Cydia]
Install ultrasn0w and reboot your iPhone 4 when prompted. Turn 3G off  (Settings -> Network -> Enable …

UPDATE : iPhone 3GS has a different security feature which disables it from using any other firmware rather than the latest available. So, if you still need Unlock and Jailbreak, custom firmware (look in categories) is the only option we have. The following article is not recommended anymore. Thank you.
Geohot rules yet again! He was the one to have unlocked the iPhone first generation phone back in 2007 and now, he is back with more! This time it is iPhone 3GS that is unlocked and jailbroken by the hacking maestro …

The Dev Team did it yet again! The cat and mouse game is on! All iPhone 3G owners (no matter what firmware you have) can now unlock their devices!

Lets get started with a step by step guide (It will be a short one this time around!)
1) Update your iPhone 3G to 3.0 . You have two options here :

redsn0w (DIY Quick!)
Pwnage Tool (Download the Custom Firmware 3.0 if you want to Jailbreak directly without messing with other tools)

Redsn0w is a revolutionary way to jailbreak, activate, unlock an array of Apple’s devices based on iPhone 3.0 firmware. Devices like iPhone 2G, iPhone 3G, iPhone 3GS, iPod Touch 1G/2G can easily be jailbroken, activated and unlocked (iPhone 2G – Automatic, iPhone 3G – Manually via Cydia & Ultrasn0w).
If you don’t want to go through the following steps and you just want the real deal, follow this guide rather wherein you can download the custom firmware file which should do the job for you!
The tool itself is so neatly packaged …

UPDATE : A new version of custom firmware 3.1 for iPhone 2G/3G/3GS is now available here!
Free your iPhone 3G 3.0 from Apple’s chain by jailbreaking your iPhone which provides you with various customization features and even unlock feature which Apple and your official carrier hate! There are various options available to achieve this level of customization on your iPhone such as download firmware files, bootloader files, jailbreak/unlock tools such as Redsn0w/Pwnage or QuickPwn.

If you are not the one to download all the various files and just want the ultimate result, …