Are there any ad networks that give publishers/websites the ability to display adverts that are chosen by the publisher immediately on page load. Lets say the page loads, and a script inserts certain key words that the ad network uses to search their inventory of adverts and assigns the nearest one with matching tags to the publisher advert area?

I know Google's adsense and many other similar services index/reads the pages content and then assign certain advert depending on the content with little or no control by the publisher...

Any help is appreciated.