MacOSXtips.co.uk has published some very handy command line tips for customising OS X.

My favourites:

5. defaults write -g NSNavPanelExpandedStateForSaveMode -bool TRUE
Sets expanded save dialogs as default (showing column/list view of folders rather than a drop down menu). Replace TRUE with FALSE to reverse.

9. defaults write com.apple.frameworks.diskimages skip-verify TRUE
Skip disk image verification. Potentially risky, use with disk images from trusted sources. Replace TRUE with FALSE to reverse.

11. defaults write com.apple.CrashReporter DialogType none
Disables the unexpectedly quit dialog that normally appears when an application crashes. Replace "none" with "prompt" to enable again.