This is the coolest thing I have seen in a very, very long time. You need to print out a piece of paper, and then hold it up to your webcam:

 
 

Here's a fun little tip: You can open most Sentex key pad-access doors by typing in the following code:

***00000099#*

The first *** are to enter into the admin mode, 000000 (six zeroes) is the factory-default password, 99# opens the door, and * exits the admin mode (make sure you press this or the access box will be left in admin mode!)

I'm not sure how prevalent they are, but here in San Francisco, Sentex building access systems seem to be the most popular.