Tutorials

JailbreakQA has jailbreaking guides, FAQs, etc. It was started by chpwn and comex, and Cydia people pay attention to it, so it's reasonably reliable and up-to-date.

GuideMyJailbreak is a new jailbreak website that walks you through a few reasons to jailbreak, some misconceptions you may have, how to jailbreak, and how to use your new jailbroken device.

Jailbreaking
You can read lots of tutorials on iClarified. Also have a look on the ground rules concerning tutorials.
 * Jailbreak 2.2.1 (For beginners)
 * Jailbreaking 3.2.x
 * Jailbreaking 4.0 and 4.0.1

Unlocking

 * TurboSIM Unlock, for iPhone 3G

Re-Provisioning (Unlocking CDMA)

 * iPhone 4 from Verizon to other CDMA Carriers using OTA (Over The Air)
 * iPhone 4 from Verizon to other CDMA Carriers using the file system

Troubleshooting

 * Entering Recovery Mode
 * Entering DFU Mode

Semi-Technical

 * Mounting the Ramdisk of IPSW in Betas 4-7
 * Understanding iPhone Backup Files
 * Change or Customize Date and Time Formats
 * Creating a NOR-only IPSW

Reversing

 * Setting up IDA Pro Advanced to reverse the baseband

Resources
Check also the link section.