It depends on your site. For adult sites, it tends to be harder to find a decently paying CPM solution. For non-adult sites, AdSense and AdBrite are great, though you will get a lot more revenue via CPC.

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If your site is geared towards generating clicks, I'd probably go with one of the CPA networks. I've been using CPALead as one of my bigger networks for a couple years now. However, they tend to have a lower pay per lead than some of the other networks. I agree with Trimmy that Adbright can be profitable. I'd also check out Maxbounty and ZML. I've been pushing a few thousand a month through both of those, and I always get paid on time.

I'd stay away from Adsense unless you're pushing pretty big traffic to your site per month. In my experience, the other networks I mentioned will earn you more money with less effort.

Those are just a couple of suggestions. I hope this helps!