Disclaimer: Don't do this if you have no choice but to break in to your own house or room. Breaking and entering into other people's homes is a felony. You can get caught.
And if you ever stumbled back to a say, hotel room only to realize your drunk roommate has passed out and locked the chain lock on the door, and no amount of knocking could get your roommate up, you can release the chain lock with a piece of string or floss.
All you need is three feet of string for this to work, and you start by looping one end of the string and then open the door as much as you can. After you've done that, loop the string around the chain with your fingers, and pull the other end of the string over the top of the door frame and unlock the door. Its as simple as that.