If you leave applications running without any open windows, Mac OS X will quit the application when it goes to the background. There is no setting in the system preferences to change this behavior. Use the following command to disable automatic application termination.
Mac OS X General
Re-enable the repeat key feature
When you press and hold a key in Mac OS X Lion or Mountain Lion, it is no longer repeated. Instead, an auto-correction pop-up would appear, allowing you to select accented characters. This default write command will re-enable the repeat key feature.
Change the frequency of software updates checks
Mac OS X 10.8 Mountain Lion removed the ability to set the frequency of checks for software updates. The default frequency is 7 days, use following command to change this:
Enable AirDrop in a wired network and old Macs
It is possible to enable AirDrop in a wired network even when your Mac is not supported. After restarting the Finder, Airdrop will appear in Finder’s navigation bar.
Add a message to the login screen
The login screen of OS X by default only shows the computer name. It is however possible to add a message or warning to the login screen. Use following command to do this.
Disable the new window animation in Mac OS X
Turn off the new window animation in Mac OS X 10.7 (and higher), new windows will appear instantly without delay.
Change default Mac OS X screencapture file type
Specify what file format you want to use for your screenshots (shift⇧ + cmd⌘ + 3) under Mac OS X. Th default format is .png, there are a lot of additional file formats available: png, pdf, tiff, pict, jpg, bmp, gif, psd, sgi and tga.