We sometimes need the serial number to check things like RAM type, Applecare coverage and other issues. There is a really easy way to find it.

Click the little Apple logo on the upper left corner of the screen. Select "About this Mac".

Under the "Mac OSX" headline, you'll see ther "Version 10.5.x". Click it.

It will change to the "Build" number. Click it again, you got your serial number.