Advertisements are the key to promotion and marketing for brands and companies. We see a lot of advertisements on televisions, magazines, outdoor, billboards, online and brands invest their creativity and ideas together to make the advertisements well enough to attract one's eye.
Brands try to target as wide audience as they can, and mediums like magazines and newspapers aren't that effective now. They either go for television advertisements or similar media, and ads on these are more interactive and expressive than Print Advertisements. The main focus is to gain customers which is only possible when the audience is targeted and if the advertisements are creative enough to increase the sales and conversions.
The ads should be attractive and conceptual at the same time. All the sites try to put ads, not all do perform good as it takes proper placement and site research. I think MangoMatter did a good job doing ad placements on their site. Users show their interest and talk about those ads which they find graphically appealing. Here's a list of one of the coolest and of course creative advertisements I found online.
NatGeo - Mundo Selvagem
Google Maps - Hello World
York ST Mechanics
Piece of Cake
Sao Paulino - Before Anything Else
Regional Flood Control District
Book Store - Brand Identity