White-hat hackers (that's the good, helpful kind) Michael Gough and Ian Robertson have created an Android app that's capable of breaking into the very popular cardkey-type door locks with a single click. It's not foolproof, since it requires some information about each cardkey system that not everyone will have, but it's still pretty amazing/uncomfortable.
The app (which is not in the Android Market, so don't even bother looking for it) is called Caribou, and relies on a vulnerability in these sorts of security systems that allows them to be unlocked remotely. It's actually a surprisingly lo-fi sort of app: You have to input the IP address of the system you're trying to hack, and then the app will perform a brute force attack (basically trying every single possible combination) until it lands on the correct one. Then the app will unlock the door for 30 seconds while you scoot inside the not-so-secure door.