Hackitivation is the process of circumventing the standard Apple activation process with iTunes. As of now you can hackitivate an iPhone in a variety of ways.
1. By using a valid AT&T sim card and connecting to iTunes. (This method will work on any iPhone. Although its not true hackitivation it can be a type of method for users without ATT.
2. Using redsn0w and not selecting any of the options. (This will work on any iPhone compatible with the redsn0w version)
3. If on 3.1.x you can run blackra1n.
4. Restore using sn0wbreeze.(Like redsn0w this will work on any iPhone compatible with the sn0wbreeze version)
5. By using a phonebook sim card and connecting to iTunes and syncing the phone.
[Note that the userland jailbreak Spirit will not hackitivate because it relies on MobileSubstrate for the jailbreak. Also note that Star will never be able to hackitivate because for someone to even use it you have to have Safari access.]