There are times when you need to locate your Mac's serial number, for example if you call Apple for support. With a laptop, that's usually pretty easy. You turn it over and it's usually right there on a sticker. Sometimes it's hidden in the battery compartment, but it's generally easy to find. On the iMac, it's sometimes less obvious, like on the bottom side of the silver leg that supports the iMac. On a MacPro or other tower, it's usually on the back. The thing is, often your MacPro or other tower will be located below your desk with a jumble of wires attached, or it may be difficult for you to lift your iMac to find the serial number. Well, there are a couple of much easier ways.
Your first option is to simply ask your Mac! Go directly to the Apple Menu (the small apple in the very top left corner of your screen) and select About This Mac... You will see a small window that tells you which version of Mac OS X you are using, how much RAM you have installed etc. Now here's the trick - click once on the version of Mac OS X and it will change to show you the build version. Click again and it will show you the serial number!
In the same window, if you now click on the More Info... button at the bottom, the System Information window will open. This gives you much more detail about your Mac, everything from memory and hard disk stats to network and device configuration and even software information. On earlier versions of Mac OS X, this was called System Profiler and it showed a list of items on the left hand side which you could click to get more information. The newer version opens directly with the Overview tab selected and you can immediately see the serial number.
So without even having to get down on your hands and knees, you now have access to your Mac Serial Number whenever you need it.