Index to pinky are: Color Club Blue Light, Orly Snowcone, China Glaze Secret Periwinkle, Sally Hansen Xtreme Wear Pacific Blue
Color Club Blue Light - This isn't periwinkle at all, it's a straight-up bright pastel blue, but I thought some might want comparison of it.
Orly Snowcone - This is a sweet, slightly-purple toned blue. I'd definitely put it in the periwinkle family. It's super opaque, and two coats are shown here.
China Glaze Secret Periwinkle - Is pretty sheer, and quite purple-leaning when put up next to Snowcone. on it's own, it's such a perfect example of periwinkle that it's not even funny. I love this one.
Sally Hansen Pacific Blue - Is a bit darker and brighter than the others but it has that gorgeous purple tone to it. I'd call this more a cornflower.