IAB Ad Unit Guidelines
The Interactive Advertising Bureau (IAB) is an organization that was set up to not only police the internet advertising world - but also to help promote internet advertising as a whole. The IAB does not make mandatory guidelines on any of the ad sizes, but it was determined that if everyone used approx. the same size ads - it would make life on the media planner and advertiser a whole lot easier. So below you will find the IAB recommended ad sizes that we try to follow:

Rectangles and Pop-Ups
300 x 250 IMU - (Medium Rectangle)
250 x 250 IMU - (Square Pop-Up)
240 x 400 IMU - (Vertical Rectangle)
336 x 280 IMU - (Large Rectangle)
180 x 150 IMU - (Rectangle)

Banners and Buttons
468 x 60 IMU - (Full Banner)
234 x 60 IMU - (Half Banner)
88 x 31 IMU - (Micro Bar)
120 x 90 IMU - (Button 1)
120 x 60 IMU - (Button 2)
120 x 240 IMU - (Vertical Banner)
125 x 125 IMU - (Square Button)
728 x 90 IMU - (Leaderboard)

Skyscrapers
160 x 600 IMU - (Wide Skyscraper)
120 x 600 IMU - (Skyscraper)

Those sizes that are bold above are part of the Universal Ad Package.