ClixSense is another program I’ve just joined. You can join for free, which is what I’ve currently done, but you can also upgrade to premium for $10.
You get paid for viewing ads, at a minimum of $0.01 per view, although there are very few available in the free section. You will also get paid for every referral who signs up, plus another bonus if they upgrade to premium, and a share of the advertising they purchase. The depth you get commissions from is greater with premium than with free.
I’ve purchased two lots of advertising here so far, both of 500 clicks, costing $6, and both for Yuwie. Referrals received were one for the first ad and three for the second (the ad was better written for the second).