It's so true that some of the ones mentioned actually aren't dupes on different skin tones/undertones. I'm pretty light, so what looks different on me looks remarkably the same on my darker friends. Anywho - here are some MAC/UD shadows that are dupes on me. (Probably a bit lighter than NW20 right now - I need to fix that!!)
MAC/UD
Retrospeck = Blunt
All That Glitters = Blaze
Grand Entrance = Sin
Mink & Sable = Chains
I also recently purchased MAC's Coconut Ice nail polish and it's very similar to Essie's Van D'Go, but Essie's isn't as watery and needs less coats. My friend also said that Essie's Chinchilly might be similar to the grey polish from the Hello Kitty collection, but we don't have the HK polish to compare.