You can stand-by for any earlier or later same-day flight on the same route without a fee.
The only exception is if you have a "time-specific" or "flight-specific" fare, where you've gotten a special deal for flying on that particular flight or time. If that's the case, then you're stuck with that one and CANNOT fly standby.
"In this present crisis, government is not the solution to our problem - government IS the problem." - Ronald Reagan
Comments made here are my own and are not intended to represent the official position of Alaska Air Group