Our first foray into Ad Servers was to install the free, open source, ad server OpenX. The product is quite powerful and has a rather mature featureset - all significant wins. I'd recommend it for any small-medium size site with the wherewithall to host an additional application who is looking for a tremendous amount of flexibility and customizability in their ad server.

If you have substantial traffic (millions of impressions/day) or are not comfortable administering your own ad server, I'd recommend Direct Media Exchange or Google Ad Manager (both are free, and quite powerful, though not as feature-rich as OpenAds in some areas).

(to be continued)