I think what you want is the ability to use a sponsored search providers xml feed for text ads and them allow you to choose which keywords ads are shown for. The way this often works is you can serve text ads via openx or any number of similar platforms and use an xml feed (there are dozens on my short list) and you can simply rotate through the highest value / highest ctr / most relevant keywords.
The tricky part is partnering with the right feed provider for what you do. Your content, your volume, your traffic's geo, your quality score all come into play.
Am I correct in what you are looking for?
If so, i might be able to point you in the right direction if i knew the aforementioned items about your site.
Yes indeed both Google and Yahoo have such services that are separate from Adsense / YPN and you must meet certain qualifications to use them. There are many alternatives if you don't qualify for those front runners. If you can share more i can steer you better.
