How can you open a locked car door without the key?

Answer

locksmith

Or if you use wireless door opening remote keyfob and you do have other one at home, then call anyone at home. When they reply to your call, ask them to take your other remote control and press open button near the phone. Put your phone next to your car when that button is pressed. Hope it works (EDIT: This is an urban legend and it DOES NOT WORK, so don't waste your time trying.)

Call a locksmith or someone who can get a spare key or remote from home.

Depending on the age of the car, a "jimmy" or removing the lock are the only options. A "jimmy" (a flat metal J shaped device that slides down the outside of the car window) can't be used on most modern cars as they are equipped with an anti-jimmy plate. A locksmith can defeat the lock and leave the door undamaged and you will need to get a replacement lock. Getting a spare key/remote from home is by far the least expensive solution.