We have a website which we want to show adverts in - However, these are custom adverts uploaded by the webmaster, not the Google adverts, or any adverts the network chooses. Ideally, there would be both options. We were considering developing our own advert-management system, but looking at the big picture, it might be better to consider other alternatives. Website is currently developed in C# / ASP.Net (Web Forms)

Are there any recommendations to some open-source delivery networks and/or external hosted advert delivery networks?

Personally I've used Google's DFP, however sometimes it is not so easy to get a Google AdSense account approved, especially while developing a new website and it not yet being launched.

Not sure if this is the best place to ask this kind of question!