How can I get back a system file after deleting it from my Mac? I've deleted a system file from my Mac and need to get it back.

System files can include:


hidden files in the root of / - especially unix files in /private 
/System files
/Library files
/Applications files that ship with the core OS


System files are not any app, user files or files that can be regenerated by a known sequence such as cache files, Spotlight index files, etc…



Canonical question and answer for this comment by bmike♦.
If you have a recent Mac then it has a recovery partition.

Power on your Mac and hold down Command+R until the Apple logo appears. This will cause your Mac to boot using the recovery partition.

Once you are in the recovery partition, you can then reinstall the operating system.

This Apple technical note, OS X: About OS X Recovery, gives more details.

You can reinstall the OS and leave your files intact. Apple removed the erase/install option so you don't have to worry about the Recovery Installer erasing your files. You now have to explicitly launch the Disk Utility tool to erase or reformat/repartition a drive when booted from recovery OS.