Propeller Ads is one of the largest pop-under networks available. They started working in 2011 and quickly became one of the largest networks with large CPM. They specialize in monetizing websites for the following niches: entertainment, videos / movies, games, dating, finance, software, gambling and more.
If you have one of these sites, you could make more money with Propeller than with Adsense. They also have banners, but pop unders works better overall. Also, if you have mobile traffic, they are a great Adsense alternative, as they work closely with ad networks that monetize mobile applications and services that lead to huge ROIs and large CPMs as well.
Propeller pays on a NET 30 basis and they have a great fill rate for countries of level 2-3. So, if you're struggling to monetize traffic from countries like India, Singapore, Turkey, South Africa, etc. Propeller has a large inventory for them.
Website: Propeller Ads