These sites won't make you rich, to be honest you probably cannot make a living on them, but they do bring in some extra cash. The cost is nothing but your time.

Whatcash has been around for several years and I've been a member since it began. They give free ads every month to Gold members. I love that perk!
http://whatcash.com/pages/whatcash.php?refid=stanne312

I have belonged to Adpaid forever. They always pay on time. It will take a bit to make the payout, but if you have a bit of spare time to click the ads they send every 24 hrs you can make a little money. Please copy and paste the url; the website is not cooperating with me right now. Thank you. :D
http://adpaid.com/pages/splash1.php?refid=avonlady

MATRIXMAILS-got to be the best out there by far. They have a great history in the PTR world and every email is worth at least a penny, plus they give you a dollar when you make just over two.
http://www.matrixmails.com/?rid=139423

Donkeymails is a fair site, they have a good number of PTC available on the website. So far this company has paid me more than the others put together, *not including Matrixmails.
http://www.donkeymails.com/pages/index.php?refid=avonlady

Peoplestring is pretty new, I just joined and hopefully it turns out to be a trustworthy site. It is a social networking site that pays, and there are several ways to make money here.
http://www.peoplestring.com/?u=leopardlady

Mylot has the same concept as Peoplestring. It is also a social networking site that pays you for blogging. I've been paid by them.
http://www.mylot.com/?ref=rosegardens

Inbox Dollars has been around for many, many years. They also have a toolbar to earn from searches, as well as send emails that are worth two cents each.  
http://www.inboxdollars.com/?r=ref8356249

BeRuby is pretty new. They pay you for visiting some websites, shopping, playing games, etc. I have made the ten dollar payout a couple of times.
http://us.beruby.com/promocode/uR4PQ1