The Social Insurance Number Code of Practice lists the federal programs that are permitted to use the SIN.
Some businesses may ask for you for your SIN. This is strongly discouraged, but it is not illegal.
Examples of when you do not have to provide your SIN:
proving your identity (except for certain government programs) applying for a job applying to rent a property negotiating a lease with a landlord applying for a credit card cashing a cheque completing some banking transactions (mortgage, line of credit, loan) completing a medical questionnaire renting a car signing up for cell phone, Internet or TV services writing a will applying to a university or college