Another relatively simple guide for TGR’s How To series, but here it is anyway: Finding your Mac’s Serial Number the easy way. All you need is, well, a Mac (and I believe you need Leopard).
In the ‘About This Mac’ menu (accessible by clicking on the Apple menu at the top left of your screen), simply click once on ‘Version 10.5.x’. This will display the build of that software. Then click again for the Serial Number. It may not be much, but I think it’s a really neat Apple “easter egg”.
Have fun finding out what your serial number is! – r.