Beware of writing sites that claim to pay writers because I have come across countless of complaints about those. However, this thread is the positive side so I guess I have to note down the first site that I have proof of payment.
ChatAbout is a writing site where you can post opinion. It is like a forum but not exactly. They have threads for topics and you post commenets that can earn you 1/2 cent for each comment. Upon reaching 1,000 comments, you can redeem that in cash equivalent to $5.