CPX Interactive have filled a niche in the online advertising world of where to send your unsold inventory from networks such as ValueClick Media, Tribal Fusion, or Burst Media. They offer reasonable rates for banners and skyscrapers ($0.20+ eCPM) for most campaigns, and then give you the option of showing low paying campaigns (normally under $0.15 CPM) or redirecting to a default network of your choice, which is a useful option.
CPX Interactive use Yield Manager as their ad management solution which offers publishers a lot of control over which campaigns they display. It also works as a dynamic online marketplace for each impression your site serves which means that you will always recieve the highest eCPM campaign available. You are also able to filter out controversial ads, and (de)select each ad individually through Yield Manager, which is also used by a number of other networks, with the inerface quite simple to use once you understand how it all works.
The level of support has always been fantastic from CPX Interactive, with response times usually under 24 hours for most questions, with their reps really helping you as much as they can on every occasion.
Overall, CPX Interactive are a reliable network, perfect for filling up unsold inventory on your site.