The most complete guide of existing brands and series in history undoubtedly used to be Stuff of Legends. This site is strongly recommended for vintage addicted.
In general, 3D painting is nowadays allowing an insane increase in brands and micro-companies that are doing just great. Considering they often only have one-off greenskinish products and not full series, keeping track is challenging, but extremely pleasing to the green twins. Finally, I still need to exclusively focus on 28mm-scale models.
For less patient searchers and yet motivated greenskins adorers, here is my personal selection:
NEW 3d Art Digital
Foundry - Goblins
Games Workshop - Warhammer
Spellcrow (see also goblins and hobgoblins)