Sure, door locks are good, but there are any number of ways to slip around them. Sometimes a little bit more security is needed when you are distracted or otherwise engaged and not ready to repel boarders. Especially when it's not your own home and castle.
I saw these on the get a cheep-cool-thingy site, Zazz, and gave them a try. This is the QuickLock, and it is a remarkably simple piece of pocket privacy protection.
The idea is simple. You slot the long face of the mechanism into the door frame, with one of the "tongues" fitted into the recess where the latch (the bit that sticks out of the door) and strike plate (the metal plate fitted to the wall the latch fits into).
You then fit the accompanying steel bolt into the closest arm of the ^ shape opening, slide it as far down (and thus, close to the door) as possible.
This then makes a wedge that holds the door in place, preventing it from opening inwards, even if the handle is turned and the latch is let free, the tongue holds against the strike plate and door-frame, and the bolt snags the door.
I'd not trust this against a solid boot, and any MOE tools other than lockpicks but for a casual traveler, who worries about someone else with keys coming in to a room you are sleeping in, or "taking a shower"
this might be just the ticket.
It folds up into a slightly more than credit card sized, but totally pocket sized kit, at 86 x 45 x 10mm and fits, without marks, in seconds.
Certainly adds a modicum of security to hostel, motel or bathroom visits you may have, and would even give the velociraptors a hard time.