For specific tutorials regarding Jailbreak, downgrading, unlock, baseband etc... follow this guide.
For specific tutorials regarding Cydia and tips, cracked and free applications, follow this guide.

• Create a DEB file
• Install a DEB file
• Grab a DEB file from Cydia
  

• Find, Download and Install IPA or this guide or this guide (cracked apps from AppStore)
• Convert a Cydia app into an IPA for iTunes
  

• Restore a Firmware (iPad / iPhone / iPad)
• Restore with Saurik Method (iPad / iPhone / iPad)

• Install ringtones from iTunes for free
  
• Make your own Ringtones with iTunes 10 (iPhone)
• Make your own Ringtones with your iDevice (iPhone)

• SSH your iDevice (iPad / iPhone / iPod)
• Secure your SSH connection (change password)

• Browse 1,000+ Themes (iPhone / iPad)
• Change your Theme with Winterboard (iPhone / iPad)
• Create your own iOS4 Theme (iPhone)
• Customize Folder and Multitask Backgrounds iOS4 (iPhone / iPad)
• Customize all icons (iPhone)
• Customize your status bar (iPhone)

• Activate Voice control (iPad)
• Disable Multitasking on your iDevice (iPhone 3G/3GS)
• Disable Wallpaper Background on your iDevice (iPhone 3G/3GS)
• Do a checkdisk of your partition (iPhone)
• Enable 720p HD Video Recording (iPhone 3GS)
  
• Enable Facetime (iPhone 3GS)
• Enable Facetime for unsupported countries (iPhone 4)
• Enable GameCenter (iPhone 3G/3GS)
• Enable HDR Photos (iPhone 3G/3GS)
• Free system partition (iPhone / iPad)
• Have two icons of the same app on the springboard (iPhone / iPad) 
• Install AndroidOS instead of iPhoneOS (iPhone)
• Install AndroidOS Froyo without any computer (iPhone 2G/3G)
• Make your apps "iPad Compatible" (iPad)
• Recover lost/deleted photos after using Spirit (old version)
• Replace a cracked screen (iPhone 3G/3GS) 
• Turn your iPhone into a Webcamera (iPhone)
• Watch Free Live TV Shows, even XXX stuff (iPhone / iPad)