It depends. If you are in the USA, in some places where you are paid a salary you are expected to never leave at 5pm, and you should be available at any time, day, night or weekend, when they need you. In some other places (like the one I work now) they are more accommodating (seems like some companies are finally getting that people are valuable) and it may be necessary at times, but not regularly.
Hourly salaries exist in USA up to some point; at a given pay rate they don't pay by the hour.
It really depends on the approach and level of exploitation that your employer has.