OpenAds offers open source alternative to Google

The free-to-download ad server is used by more than 25,000 websites according to the OpenAds website. As the above graphic illustrates, OpenAds allows you to manage ads from multiple network sources (Google, ValueClick, Advertising.com and so on), among other features typically associated with high-priced ad server applications.

OpenAds appears to be focused on providing self-service options for websites, as opposed to larger providers (DoubleClick, Advertising.com, etc.) which are geared to serving the higher end of the market.

Interestingly, the most popular ad network among OpenAds users is Google AdSense -- likely the closest competitor in terms of service offerings.