Well, there's good news and bad news. The good news is elevators have tons of ways to make sure this doesn't happen. An elevator failure on this magnitude is incredibly unlikely because it would require multiple systems to fail. In fact, most of the above video is spent explaining all those systems and how, even if they fail, there are still more measures in place that likely stop the elevator from a complete free fall.
While movies portray the elevator drop happening more than it does in real life, you still want to know what happens. That's the bad news. There are a ton of variables -- including the height from which you're falling -- but if your elevator really is in free fall, it's bad. The old myth about jumping at the last second might work, but the amount of strength and timing required means it's close to impossible to do it. (As Mythbusters showed in this mini-episode from years back.)