I like to use the Mactracker app, although it doesn't list the EMC numbers which seem to be favoured here on ifixit
on the other hand, Mactracker does list the "Machine ID" code and the Apple style general names, for example :
"iMac 11,1" which is what Apple refers to in their documentatio as a: 27" iMac (Late 2009) Core i7 machine. These latter style descriptors are what you will find in some of Apple's support and discussion group sites and other places.
also the machine ID (e.g. iMac 11,1) is what shows up in System Profiler on each Mac, so personally I find this more useful than the EMC number which is only physcally stamped on the machine (and is sometimes worn out!). However I think the machine ID numbers sometimes apply to a family of machines of same type but different CPU speeds
I wish ifixit would additionally add the Machine ID numbers to the model id info :)
while I'm at it I may as well mention Apple's own page for identifying Mac models