http://www.streamads.com/

i don't know how active this is, but it looks interesting, a bit like openX for radio. does anybody have experience with this service?

from the website:

The StreamAds™ platform is a one-stop solution for all your online radio advertising needs.

It's all you need.
Manage from one central web interface:

* Pre-roll audio/video (aka gateways)
* In-stream ad-replacement (with synchronized display ads)
* In-stream 1-to-1 targeted advertisements (Audio and even Video)
* Banners ads
* Text advertising
* Surveys

Business as usual: Works with what you already have
StreamAds™ works with all major automation systems (AudioVault, SAM Broadcaster™, Maestro, Scott Studios, and RCS to name just a few).

Manage a single station or a network of stations from one central web interface with multiple levels of access for your staff.

Increase revenue
Increase your revenue potential by joining 3rd party provider programs that go out and sell your unsold inventory.

Maximize your value with our unique 1-to-1 targeting technology that allows each listener to receive a unique targeted advertisement.

Customize
Use our Player SDK for rapid media player development or integration into your existing media player - or have Spacial Audio™ design a custom player for you. Our flexible Management API and Delivery API lets your developers tie directly into StreamAds™ and take control of the entire process, if required.

Powering the revenue at leading companies for over 6 years
StreamAds™ has stood the test of time. It's been used by hundreds of terrestrial radio stations and thousands of online stations for the last 6 years.