It depends a lot on what you do online, where (if at all) you shop online and what rewards you want.

Milespree and Giantrewards seem to have about the highest payouts on shopping, but Milespree has no small rewards. MyPoints used to have about the most freebies per month, but don't pay nearly as much back on shopping.

Many sites have ways to earn points from clicking on ads, etc, but some of them (eg. Blink) are worth very little each. If you had unlimited time to waste, it might be worth it!

One of the best value sites is Etour. You get 80 Etour points per day from visiting just 4 sites, and I have actually found some really useful sites there, not just the usual crud that everyone else has.