Clixsense is still the oldest, most top searched, and World's No.1 PTC site.
It has been paying since February 2007. If you'd like to enter the Paid To Click industry, this site is the first thing you should try.
Sweeva is a unique traffic exchange site. It has probably the cleanest interface from all other traffic exchange websites.
The quality of traffic is superb and it allows you to interact with the people who view your site. I'd rate this a 10.