A few tweaks for changing OSX 10.7ish, most of these will not work on 10.8+.

Verbose booting

    % sudo nvram boot-args="-v"

Credit: OSXDaily.com

Prevent .DS_Store on networked drives

    % defaults write com.apple.desktopservices DSDontWriteNetworkStores true

Credit : MacOSXHints.com

Get rid of the 3d dock

    % defaults write com.apple.dock no-glass -boolean YES; killall Dock

Credit: MacOSXHints.com

Disable dashboard

    % defaults write com.apple.dashboard mcx-disabled -boolean YES; killall Dock

Credit: MacWorld.com

Safari debug menu

    % defaults write com.apple.safari IncludeDebugMenu 1

Credit: MacOSXTips.com

I found a ton more on OSXNotes.com, check it out there are some really interesting changes that can be made.