OpenX Ad Server has the ability to

OpenX Ad Server has the ability to track individual website visitors by using cookies (small text files which are saved on the user's temporary internet files). When a user opens a website, a session starts. When the user closes the website and the browser, the session ends.
Suppose that a visitor came to the website where our campaign banners were shown using a web browser and he/she browsed five pages there and has already seen three of the same or different banners from the same campaign. The fourth banner will not be shown from this campaign for the user session. It is possible that on the next day, a maximum of three banners will be shown to the visitor for another session (starting a web browser again). At the end, this visitor will not see any banners from this campaign after nine views as it is the total limit in our example.
If the user browser doesn't save the cookies information (cookies are disabled), OpenX neglects Delivery capping per visitor settings. If the visitor deletes the cookies after some time (it is generally done by clearing temporary files in browsers such as Firefox, Internet Explorer etc.) the Delivery cappings are reset to zero.
It is wise to use the Reset view counters after option if there are major changes in campaigns and visitors are expected to click on a new set of banners. For example, if we set 240 hours counter, the visitor delivery capping limits will be set to zero for every 10 days, thus increasing delivery chance for a certain visitor.