1. An article comprising a non-transitory, tangible machine-readable medium embodying instructions, that when executed by a computer system cause the computer system to perform operations comprising: transmitting over a network, using a user browser, a request for a web page to a first server; receiving over the network the requested web page into which an ad is to be inserted; receiving, by the browser, a script from a second server; requesting bids, by the browser using the script, from a plurality of bidder servers by sending messages to the plurality of bidder servers, the messages soliciting one or more bids for presenting an ad by the browser; providing information by the browser to at least a portion of the plurality of bidder servers, the information including at least information about the web page, into which the ad is to be inserted, receiving, by the browser, one or more responses from responding bidder servers in the plurality of bidder servers, the one or more responses including respective bids; evaluating, using the script, the responding bidder servers' bids and providing an evaluation result, wherein evaluating includes assigning an adjustment factor to at least one of the bids, and the adjustment factor is assigned based at least in part on discrepancies in a monetary value of impressions delivered to at least a first responding bidder server and a second responding bidder server; selecting, using the script, one of the bids based at least in part on the result the evaluation; causing a notification to be transmitted by the script at the browser to the ad server that submitted the selected bid; receiving by the browser, from the ad server that submitted the selected bid, a link to a first ad; and accessing the first ad using the link.