A self-defence course will teach you how to use your keys as a weapon, but in practice, when you're under attack and panicking, protecting yourself with a keyring isn't as easy as it sounds. So the Hit, from Unikia, combines a loud audible panic alarm with a pop-up holder that turns a key into a switchblade that's easier to wield as a weapon.
The Hit's unique design actually makes it incredibly difficult to silence the alarm if you don't know the secret to deactivating it. So you'll want to encourage your friends not to mess around with it. And if the alarm doesn't scare off a would-be attacker, you can do a considerable amount of physical harm with the key that automatically pops up when the alarm is activated.
If you live in a sketchy neighbourhood, this could be the best £20-odd you ever spend on a keychain. And here's to hoping you can actually use that key to open a door without setting off the alarm. [Unikia]