Sinful Colors has several polishes that seem to be only good for layering. A prime example of this is Let Me Go - I bought three of these thin polishes, and nothing seemed to get them to opacity. But they flash some amazing colors in the bottles! Let Me Go looks gold, purple and blue in the bottle.
I layered this over Sinful Color Verbena, and wow. What a stunning color! The blue flash turns into a green shimmer. The purple is kind of absorbed by Verbena, but you can see the gold shot through, I think, in these photos. One layer of Verbena, one layer of Let Me Go here.
The other two polishes that I have that are duochromes and great for layering are Bali Mist and You Just Wait. If you have these, and don't know what to do with them - give layering a whirl, you'll be pleasantly surprised!