You can listen to lectures of professors from world leading universities for free, and perform tasks too, but if you want to get a certificate, it can cost money.
The exact conditions depend on the specific course; there are more than 3500 of them in the catalog.
Free Courses Credential Key
CC = Certificate of Completion
CA = Certificate of Accomplishment
HCC - Honor Code Certificate
VC$ = Verified Certificate
VCA$ = Verified Certificate of Accomplishment
SA = Statement of Accomplishment
SP$ = Statement of Participation
CM = Certificate of Mastery
NI - No Information About Certificate Available
NC = No Certificate
Below are links to some of them:
Table of Contents