From Kathleen Crislip,Your Guide to Student Travel.
I'm an adventurous traveler, and I've certainly stayed in some hotels/motels/hostels where my safety in my room was questionable whether because of the neighborhood or the iffy door lock. Check out these products that will bring you some peace of mind - a portable door lock which works anywhere and a doorjam with alarm that shrieks like a banshee if someone tries to get into your room uninvited.
1. Portable Door Lock
Awesome device - this portable door lock works like this:
Squeeze and slide lock back (illustration exists on the packaging)
Insert hook into door strike plate (again, package illustration shows you how to proceed)
Slide lock forward and you're safe!
2. Wedge Guard Door Stop Alarm
From the manufacturer: "The Door Stop Alarm is a unique burglar alarm. The Door Stop is designed in the shape of a 6" long door wedge. Simply place it behind any door. When the door is opened it will jam the door and at the same time give off a loud, shrill pulsating alarm...this is a completely self-contained, easy to carry and easy to test unit. 118 dB alarm, anti-slide sole plate, uses one 9 volt battery (sold separately). 7" x 2" x 2-1/4"." Needs two AA alkaline batteries.
This comes from About.com -student travel