Force the printer icon to auto quit after a print job completes. Right-click on the icon and select “Auto Quit” or do:
defaults write /Users/yourusername/Library/Preferences/com.apple.print.PrintingPrefs "Quit When Finished" -bool true

Disable disk checksum verification
defaults write com.apple.frameworks.diskimages skip-verify true

Gradient highlights for Stacks
defaults write com.apple.dock mouse-over-hilte-stack -boolean true

Add recent applications as a Stack on Dock
defaults write com.apple.dock persistent-others -array-add '{ "tile-data" = { "list-type" = 1; }; "tile-type" = "recents-tile"; }

Make extended print dialog show by default
defaults write -g PMPrintingExpandedStateForPrint -bool true

Change the current screenshot format
defaults write com.apple.screencapture type jpg

Disable iTunes store arrows
defaults write com.apple.iTunes show-store-arrow-links -bool false

Stop Safari “saved text” dialogs
defaults write com.apple.Safari DebugConfirmTossingUnsubmittedFormText NO

Force Safari to open all links in new tab
defaults write com.apple.Safari TargetedClicksCreateTabs -bool true

Disable application crash dialogs
defaults write com.apple.CrashReporter DialogType none

Make hidden Dock icons transparent
defaults write com.apple.Dock showhidden -bool true

Disable Dashboard
defaults write com.apple.dashboard mcx-disabled -boolean true

Stop disk image verification
defaults write com.apple.frameworks.diskimages skip-verify true

Enable Data Detectors in 10.5 iChat
defaults write com.apple.iChat EnableDataDetectors true

Modify Sheet Animation
defaults write NSGlobalDomain NSWindowResizeTime .001

Login window tweaks

The text underneath the “Mac OS X” title of the login window will change if you click on it. You can configure the login window to display any of these as default. Valid options are HostName, SystemVersion, SystemBuild, SerialNumber, IPAddress, DSStatus, or Time
defaults write /Library/Preferences/com.apple.loginwindow AdminHostInfo Time

You can add a message to the login window. This is useful in a lab environment when you need to give instructions to your users.
defaults read /Library/Preferences/com.apple.loginwindow LoginwindowText "Some custom message."