Kevin Vigneault, Project Manager at VigetAdvance has post a great article about banners placement on a heat map.

The heat map helps to visualize the current trend in banner advertising. It shows banner size and placement (highlighted in white) on 50 different popular sites. The brightest white areas are where banner ads were most frequently placed and the black areas are where no banner ads were displayed. The two red lines are 1,024 pixels apart to provide scale.

His conclusions are:

• The most frequently used areas are along the top and right sides of the page.
• The most popular ad sizes are 728×90 Leaderboards and 300×250 Medium Rectangles.
• More sites place Leaderboards above the main navigation than below.
• Very few sites are placing banner ads along the left side of the page.
• The observed common placements do not match up very closely with Google’s suggestions.