If you're talking about something being required:
* Optional
* Voluntary
* Elective
* Non-mandatory
* Discretionary
If you're talking about something being forced:
* Free
* Unforced
* Voluntary
* Uncoerced
Example:
* Compulsory: All students must take math class.
* Optional: Students can choose to take art class.
Example:
* Compulsory: The government requires all citizens to pay taxes.
* Free: People can choose to donate money to charity.