The next time you're travelling and need an easy and portable door lock, try using a broken metal fork.
Of course, most places you'll stay already have a lock. But if you're staying at a friends or at a bed and breakfast, you may find your door doesn't have one (the last Airbnb I stayed at didn't, for example).
This lock only works when you're in the room and won't prevent a forced entry, but it's helpful to prevent accidental unwanted guests while you're in your room. Check out the video to see it in action.
Dinner Fork Door Lock [YouTube]