There used to be an open source self-hosted (php) ad application called openx. It worked well for quite a while. In its later years, it suffered a number of high-profile security vulnerabilities, and the open source version was poorly maintained since OpenX [the company] was focused more on their hosted solution which probably had migrated to a different codebase or at least was a major version past the open source codebase.

The open source version has been renamed "Revive Adserver", and it looks maintained, but I don't think it's used nearly as much as the openx [open source version] of old.

If you use Revive Adserver or you design a server-local ad system in-house, it won't be as sophisticated as gigantic ad-providers who can do all sorts of segmentation and analysis (producing pretty reports which execs and stakeholders love even if that knowledge adds no value to the business).