Mactracker
Mactracker is a free utility that, amazingly, contains a complete listing of all the makes & models of Apple computers and Macs ever made. It gives their variants, lists their limitations, gives precise data on RAM configs, and provides a host of other data about your computer(s) which can prove useful as you consider an upgrade or think about buying a used one.
It's updated about twice a year, and you can download it for free from the URL below.
Drew Janssen recommends Mactracker to every Mac owner, because it's a great help when you want to know more about your Mac.
iPhone version
There is also an iPhone version of the app on the iTunes App Store that is invaluable when you're on-site dealing with multiple generations of the same model Mac.
Credits
Originally contributed on Mac-Min by Drew Janssen, October 20, 2008. iPhone version contributed by David Burke, December 14, 2010.