All the Coursera courses I've looked at can be audited for free. There are some advanced features of the courses that require enrolling for a fee. If you enroll

All the Coursera courses I've looked at can be audited for free. There are some advanced features of the courses that require enrolling for a fee. If you enroll, and pay the fee, you also get a Certificate of Completion. If you cannot afford the fee, you can apply for financial aid.