An ad server is a computer server, specifically a web server, that stores advertisements used in online marketing and delivers them to website visitors.

Ad servers come in two types: local ad servers and third-party or remote ad servers. 

Free Ad Servers:

Open X - OpenX is a piece of free software that allows medium and large website to control their inventory.

Google Ad Manager - Google Ad manager is built specifically for small publishers. You can only use this ad server for small sites, because it is not compatible with any larger ad servers, and you have to put ad code straight into the header of your site.

Corporate Ad Servers:

Zedo - Zedo is great but they were very expensive compared to all the other companies. This might due to the great customer service and the results they can bring your sites.

DoubleClick - Instead of Trafficking, reporting, billing and account details all in one sections, they seem to have segmented them. They are the worlds best ad serving tool and the most used.

Atlas - From Microsoft. They don't seem to offer good customer service.