iPhone, an extremely popular multimedia and Internet-enabled quad-band GSM EDGE-supported mobile phone from Apple and AT&T, normally does not allow access to its media player and web features unless it has also been activated as a phone through AT&T. In other words, subscribe and sign the service contract with AT&T and pay mobile service provider monthly fees. A hacker named Jon Lech Johansen managed to crack and bypass this restriction and AT&T service requirement and thus unlock the iPhone iPod and WiFi features.
The original hack involves manual cracking steps using PAS (Phone Activation Server) is pretty much complicated, and involves usage of advanced custom software to modify iTunes binary. iActivator is a GUI utility based on jailbreak, iASign, and iPhoneInterface (all tools to crack iPhone) which allows users to easily activate iPhone from Mac OS X graphical user interface. With iActivator, users can activate iPhone without AT&T service and can subsequently use the phone as broadband wireless or WiFi Internet surfing device, PDA and iPod. But you can use it as phone to making or receiving calls.
Note that iActivator is also not a SIM lock unlocker for iPhone.
Download iActivator with complete instruction on how to use here. The brief guide is that connect iPhone to computer, run iActivator and perform jailbreak to put iPhone into a state where the files are allowed to be written to. Then press and hold the home button and sleep/wake button for about 25 seconds until the window in iActivator goes away. Subsequently a new PEM certificate file will be put on iPhone to make iPhone allows activation. Finally step is generating activation .plist file and using the .plist file to activate iPhone. Of course, you shouldn’t forget to jail back iPhone to put it back to read-only state.