As banner designer I follow the IAB's industry standard technical specifications for online display advertising that (Belgian) agencies have to follow regarding weight, play length, number of loops and Clicktags.

I am aware of the existence of the different formats (IMU's, skyscrapers, etc...) and of the different brands of Clicktags that often confuse banner creators and clients alike because of Flash's case sensitivity. I sure hope the universal Clicktag is here soon to solve that mess. Banners created to be used directly on websites owned by the clients of course do not have to follow these strict guidelines.