The Alexander Wang: pocket tank
Alexander Wang tank in a casual style
Blake Lively dresses the tank up a bit
Though if you can not afford the $75 - $135 price tag, no need to worry I have found some similar styled tank tops for cheap.
If you are looking for a dress that is sexy and stylish with this style, check out this dress worn by Lara Stone in Calvin Klein. (picture from Harpers Bazaar; Top 10 best dressed.