COO Families
This limelight and guide will cover the major variations of the figure initially.
As this guide grows, each figure you browse may have more information and variations, so please click the links as you find them to learn more.
Since many things can happen during manufacture, there are too many possible minor variations that I could never fully document all of them without having seen hundreds to thousands of examples in my hand. That won't happen unless I hit the lottery.

At my last count my loose collection has 33 figures, somewhere between 16 and 18 of which I consider major variations. (I need to check again!)
Working off the mold pair concept, there are at least 12 COO stamp variations and possibly more.
I will also try to show the minor variations that exist for each figure if I have examples available.
For a good read on the basics of COOs, molds and variants, you should visit WOLFF's (wbobafett) Introduction to COOs.
psybertech's COO & Family Guide
This is based off of wbobafett's (Wolff) awesome COO Guide on TIG and Wolff's COO Guide at the SW Space Club.

The above guide and the one below shows an overview of all the major variants by family. This above is using the COO stamps on the figure's legs, the one below is just based on heads.

Each family will have a link below if you wish to see more information on the figures inside each family.
Family #1 - 4 Line Hong Kong - AKA COO#1
Family #2 - 2 Line Hong Kong "Straight" - AKA COO#2
Family #3 - 2 Line Hong Kong "Slanted" - AKA COO#3
Family #1 Altered - 4 Line Raised Bar China - AKA COO#4
Family #2 Altered - 1 Line No COO "Straight" - AKA COO#5
Family #3 Altered - 1 Line No COO "Slanted" - AKA COO#6