Small personal alarm emitting a loud noise (130 decibels, apparently), to alert help scare off an attacker if you're attacked. Often provided free to employees of large organisations and university students.
Mine is a 'Walk Easy' and also gives off an ultraviolet tracer spray, to help the police find the bastard who attacked you. They have a web site, if anyone wants a rape alarm and can't find one:
http://www.personal-alarms.co.uk/