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