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 …

iPhone 4, despite its signal woes is a beast when it comes to processing power and pure, raw speed (a lil’ better than 3GS). Jailbreaking iPhone 4 will give you access to themes, productive tools, unlock and more. I tested this approach with a 3GS, 3G and iPhone 4. It is a multi-iPhone jailbreak and requires no computer access! It is the same as described here but I wanted to document Cydia apps for iPhone 4.
Initially, I was unsure if Cydia is even needed for iPhone 4 with multitasking, wallpaper …

Apple’s iPhone OS 4.0 is close to revolutionary if not game-changing. Some of the new features being Multitasking (3GS only), iAds (ads with fluff!), Game Center (openfeint?).
Watch iPhone 4.0 Keynote in HD quality here.

Multitasking
Jailbreak folks have been enjoying the much touted multitasking feature for years while Apple finally makes it official, their style.
If you have Cydia on your iPhone, try Circuitous, a free multitasking app with better features than that of Apple’s!

Folders
You can now categorize your apps and put them in folders for quick and easy access. Again, this is something …

Apple’s iPhone had little or no updates to the UI ever since its inception (2007 June). The homescreen is way too boring when compared to the ever so popular Android phones. To add some life to the existing iPhone homescreen, install a free application called ‘Element’. It is a Cydget widget. You no longer have to launch each individual app (weather, email, sms, calendar,.. updates). Following are the steps to install it on your iPod Touch / iphone.

Your iPhone should be jailbroken before proceeding.
Launch Cydia and search for ‘Element’ and …

Apple tightened security with iPhone 3GS and the latest 3GS hardware version is only capable of tethered jailbreak 🙁
Saurik, the man behind Cydia implemented a clever approach to bypass Apple’s insane security features of the 3GS! He explains the whole process in great detail here.
In short, all you have to do is :
Jailbreak your iPhone 3GS. Trust me it is easy, fun and super productive to jailbreak your iPhone and it is perfectly legal!
Launch Cydia on your iPhone 3GS
Click on ‘Make my life easier, thanks!’. If you don’t see it …

UPDATE : The issue has been fixed in the latest update released. Launch Cydia and update GV Mobile to v1.22.2 and you should be all set!
Sean Kovacs’ GV Mobile is no doubt the best native iPhone client available only on Cydia after Apple’s asinine decision to block all Google Voice related apps from the AppStore.

GV Mobile App suffered from some issues after Google Voice changes on the web front end. The two known issues being :

Invalid Credentials Error
Login Info not being saved (requiring username/password each time the app was launched)

Solution …

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, …

Backing up Cydia apps has always been a pain! Different implementations exist with little or no success. I recently lost all my Cydia Apps since I thought I backed them up just fine (using AptBackup which can be installed from Cydia). How to save hours trying to get back all the apps installed in Cydia? And here goes the step by step guide to achieve it :
Test environment : I have used AptBackup while restoring iPhone 3G from 2.2 to 2.2.1 firmware on a Mac. I used iTunes 8.0.2.
And here …

Go ahead and follow this if you have a jailbroken iPhone 3G (Firmware below 2.2.1) and don’t want to mess around with the unlocking tools for 2.2.1 firmware.
Follow the following steps for both Windows and Mac Operating systems :
1) 2.2.1 Firmware from Apple iPhone 3G updates the baseband to 2.30 disabling the software unlock.
2) Make sure you have jailbroken your iPhone 3G Firmware below 2.2.1 using QuickPwn 2.2.
3) Now,if you have access to a Mac, feel free to download Pwnage 2.2.5 and build a custom firmware. Windows users might want …

Those of you who want iPhone 3G 2.2.1 firmware with 2.28 Baseband and Cydia, download both the files below and join them using hj split.This works both on Windows and Mac. 2.2.1 may not be an important upgrade but it definitely puts and end to accidental upgrades to 2.30 baseband.

Please make sure your iPhone 3G is already pwned (you should have Cydia installed!).
Launch iTunes (Windows or Mac) and restore your iPhone to the downloaded Custom Firmware. (Hold Shift key (Windows) / Opt key (Mac) whle hitting the restore button in …