In order to show relevant ads to a user I just put Google code on my site and they target the ads based on their knowledge of that user. Is there anyway I can use AdSense or any other advertising company to decide myself what ads to show to the user from their pool? i.e. I will decide to show him a "technology" related ads?
Search API for Shopping can let you search and display ads from Google Affiliate network, and the products offered from merchants. Google Affiliate let you choose Merchants of your choice and you can add emended id links in any kind you ads you like, you can put your own designed Ads and add you publisher link in it, they have hardware/software, telecommunication etc like categories where you can choose Merchants of you choice.
Adsense and its api won't going to help you i guess, as adsense api is for revenue sharing etc. you can't edit or choose ads either.
AFAIK, Google's AdSense ads don't target specific users. They display ads on Gmail like that, but all other ads are based on the content of the page not the user.
If you want to show more relevant ads to your users, you can do audience profiling and segmentation through companies like Rapleaf, Quantcast, Full Circle Studies (ScorecardResearch), Brilig, and I think Alexa also offers marketing analytics.
The only company I know of that does user-specific targeting is Rapleaf, which has an API that lets you query user information based on email addresses to deliver advertising targeted at that specific user or to personalize their user experience.