Your Mac's serial number is required for warranty service, among other benefits that Apple offers its customers. Therefore, while not commonly used, you may occasionally need to look it up. While you can always look on your Mac's chassis to find the printed label that contains the serial number, there are a number of additional ways do do this, some of which are more convenient than others.
Besides using the Mac's chassis, the easiest way to find your Mac's serial number is to use the About This Mac information panel that's in the Apple menu. Choose this option, then click the gray version string for your installation of OS X. This will toggle the string between showing the OS version, the OS build number, and your system's serial number.
The next option is to use the System Information utility's "About" panel, which is available by clicking "More Info" in the "About This Mac" panel, or by opening the tool in the Applications > Utilities folder, then pressing Command-i. With this panel open, you can see your Mac's model number and some hardware information in the Overview section, along with your Mac's serial number.
The system information tool also generates an in-depth system report. This is done by clicking the "System Report" button in the About panel, or simply by opening the System Information tool directly from the Finder. In the report, you can click the "Hardware" section (this is the default section), then in the "Hardware Overview" you should see the serial number listed, as well as with other details such as processor, memory, and cache information.
The System Information utility also has a relatively obscure option that will speak the serial number to you. As with many applicaitons in OS X, you can highlight the serial number in the Hardware Overview list, then right-click the selection and choose the option to speak it; however, this will speak the serial number somewhat swiftly. Instead, simply press Command-4 in the System Information tool, and the default text-to-speech voice will speak the serial number at a slower pace.
The final option is to look up the system's serial number in the OS X Terminal, which may be useful if you are logged into a remote system via SSH. To do this, you will need to use the "system_profiler" command. This command when used by itself will generate the same lengthy report that's produced when you open the System Information utility, so used alone it may offer too much information. However, you can limit its output to showing only hardware information (which contains the system's serial number) by running the following version of the command:
This command can be further refined by piping its output through "grep" and filtering it to only show lines that contain the word "Serial," by the following command:
system_profiler SPHardwareDataType | grep Serial
With these options, you should be able to locate your Mac's serial number, which is useful for verifying eligibility for warranty service or other assistance offered by Apple.