Many courses offer certificates, but they also require you to pay for the certificate. At the moment, no major MOOC provider routinely offers a free certificate, although FutureLearn offers a free upgrade on some courses. See 130 FutureLearn Courses That Still Offer Free Certificates.
Many course providers offer a free trial or a money-back period after payment. You may be able to pay for the certificate after starting the course. Check the course provider's information pages carefully for these details.
In addition to individual course certificates, some providers have created their own certificate programs. These programs are something between a single course and a full degree, and they usually involve taking a sequence of courses. Examples of these would be Coursera’s Specializations, edX’s XSeries, or Udacity’s Nanodegrees.