OpenX is the company that sponsored Matthieu to start building the open source software Piwik. Matthieu stayed employed at OpenX for more than a year, mostly developing Piwik and supporting the community.
The OpenX ad server is a free open source ad server which you can download and install on your server of choice.
You can use it to manage banners (self-made or provided by ad networks) of as many different types as you like, across as many websites as you like. It provides options for banner capping, user- and site-based targeting, time- or impression/click-limited campaigns, and the prioritization of campaigns.
You can review the statistical performance of a specific banner, campaign, advertiser, publisher or zone, and re-prioritize your banners as needed.
For more information, screenshots and a demo video, have a look at the Piwik blog post: If you need to serve ads on your website, check out OpenX Ad Server 2.8!