What can you substitute for triple sec in a margarita?

Triple sec is an orange-flavored liqueur that is commonly used in margaritas. If you don’t have triple sec handy, you can substitute with another good-quality orange liqueur like Grand Marnier or Cointreau. 

For a non-alcoholic substitute you can add fresh orange juice, or agave nectar/simple syrup, to add a touch of sweetness.

What can you substitute for triple sec in a margarita?