Ask HN: Finding an ad network for my startup (other than Adsense)

I've used Adsense in the past, but won't be able to use Adsense in my current startup due to Google's terms of use. (My app is stateful; When users click an ad, I need the advertiser's site to open in a new window so that users don't lose their place in my app. Google doesn't allow that.) I have applied to Yahoo's publisher network a few times but never received a response; I put in a request for Microsoft's ad beta program today. My startup will be freemium; I'm not solely relying on ads. Any suggestions for a reasonable Adsense alternative?