SpringBoard
Calculator Icon - /Applications/Calculator.app/icon.png
Maps Icon - /Applications/Maps.app/icon.png
Calendar Icon - /Applications/MobileCal.app/icon.png
Mail Icon - /Applications/MobileMail.app/icon.png
iPod Icon - /Applications/MobileMusicPlayer.app/icon.png
Notes Icon - /Applications/MobileNotes.app/icon.png
Phone Icon - /Applications/MobilePhone.app/icon.png
SMS Icon - /Applications/MobileSMS.app/icon.png
Safari Icon - /Applications/MobileSafari.app/icon.png
Camera Icon - /Applications/MobileSlideShow.app/icon-Camera.png
Photos Icon - /Applications/MobileSlideShow.app/icon-Photos.png
Clock Icon - /Applications/MobileTimer.app/icon.png
Settings Icon - /Applications/Preferences.app/icon.png
Stocks Icon - /Applications/Stocks.app/icon.png
Weather Icon - /Applications/Weather.app/icon.png
YouTube Icon - /Applications/YouTube.app/icon.png
Dock - /System/Library/CoreServices/SpringBoard.app/SBDockBG2.png
Locked Screen
Unlock Text - /System/Library/Frameworks/TelephonyUI.framework/bottombarlocktextmask.png
Slider Bar - System/Library/Frameworks/TelephonyUI.framework/bottombarknobgray.png
General
Ringer background when you use volume keys - /System/Library/CoreServices/SpringBoard.app/hud.png
Badge (alert bubble for calls, sms, etc.) - /System/Library/CoreServices/SpringBoard.app/SBBadgeBG.png
Top info bar - /System/Library/CoreServices/SpringBoard.app/FST_BG.png
Battery Charging Screen
Battery Background - /System/Library/CoreServices/SpringBoard.app/BatteryBG_1.png-BatteryBG_17.png
Top Info Bar (signal strength, battery, etc.)
Carrier Logo
lighter one - /System/Library/CoreServices/SpringBoard.app/FSO_CARRIER_ATT.png
darker one - /System/Library/CoreServices/SpringBoard.app/Default_CARRIER_ATT.pn
Signal Strength
/System/Library/CoreServices/SpringBoard.app/FSO_0_Bars.png-FSO_5_Bars.png
/System/Library/CoreServices/SpringBoard.app/Default_0_Bars.png-Default_5_Bars.png
Bluetooth - /System/Library/CoreServices/SpringBoard.app/FSO_Bluetooth.png
Background bar color HomeScreen - /System/Library/CoreServices/SpringBoard.app/FSO_BG.png
Background bar color Lock mode - /System/Library/CoreServices/SpringBoard.app/FST_BG.png
Background bar color for all other screens (Text, Safari, Mail, etc.) - /System/Library/CoreServices/SpringBoard.app/default_BG.png
WiFi Bars
/System/Library/CoreServices/SpringBoard.app/FSO_0_AirPort.png
/System/Library/CoreServices/SpringBoard.app/FSO_1_AirPort.png
/System/Library/CoreServices/SpringBoard.app/FSO_2_AirPort.png
/System/Library/CoreServices/SpringBoard.app/FSO_3_AirPort.png
/System/Library/CoreServices/SpringBoard.app/Default_0_AirPort.png
/System/Library/CoreServices/SpringBoard.app/Default_1_AirPort.png
/System/Library/CoreServices/SpringBoard.app/Default_2_AirPort.png
/System/Library/CoreServices/SpringBoard.app/Default_3_AirPort.png
Power on Screen
The logo - /System/Library/CoreServices/SpringBoard.app/applelogo.png
Power Off Screen
Power off slider - System/Library/Frameworks/TelephonyUI.framework/bottombarknobred.png
Calculator
Background - /Applications/Calculator.app/CalculatorBackground.png
Screen Background - /Applications/Calculator.app/LCDBackground.png
iPod
Rating Stars - /Applications/MobileMusicPlayer.app/star_filled.png
Phone
Keypad - /Applications/MobilePhone.app/BarDialer_Sel.png
SMS
Input Field (Bubble where you type you’re text message) - /Applications/MobileSMS.app/BalloonInputField.png
Balloon 1 (Bubble where sent text messages are) - /Applications/MobileSMS.app/Balloon_1.png
Balloon 2 (Bubble where received text messages are) - /Applications/MobileSMS.app/Balloon_2.png
Bottom Bar Background (Background behing the text input field) - /Applications/MobileSMS.app/MessageEntryBG.png
Safari
URL Bar (Idle) - /Applications/MobileSafari.app/Url.png
URL Bar (Inactive) - /Applications/MobileSafari.app/UrlInactive.png
URL Bar (Active) - /Applications/MobileSafari.app/UrlProgress.png
Mail
Trashcan - /System/Library/PreferenceBundles/MobileMailSettings/trashmbox.png
Please feel free to add more to the list.