Internet & Tech: What sequence models are used to serve online ads?

Does anyone know the different sequences in which online ads are served?

For example the few things that come to mind as a firm like Yahoo or Google deliver ads, they must figure out the best way to show the ads by either:

1. Random order
2. Scheduled (eg. at 1PM show ads 1,2, 2, etc)
3. Sequential (eg: ad 1 was shown, now show ad 2)

I'm currently developing an ad server application that I plan to use on a network of media sites (forums, chat, online radio, magazine, online TV) that I'm building.

This ad server app, will be located on a centralized location where all these sites will be able to consume ads. The ads will be in different sizes, mostly in flash or jpg format. The easiest way I can serve the ads could be random, but I'm looking for a more systematic (and possibly predictable) way of doing it.

Any ideas or sources you can point to on this matter would be greatly appreciated.