I have tried to separate the PTC (Paid to Click) sites from the PTR (Paid to Read) sites, though there is often some crossover between the two types. These are generally the easiest to earn money on - in the case of PTC sites, you click ads, usually for up to $0.01 per ad or less, and many offer great benefits for bringing on referrals. In most cases, if you upgrade, you can earn more both for your clicks and your referrals' clicks.
ClixSense
http://www.clixsense.com/?4243124
Neobux
http://www.neobux.com/?rh=6272696B656E