Meet the Key: a compact self-defense tool that can buy you time when attacked. Created by Phil Strenkowski, a Martial Arts black belt, the Key is a key-chain you can carry with you everywhere. You can use it deliver blows to the head, hand, or other pressure points of an attacker to buy yourself enough time to get away.
The Key may not be a real weapon but is powerful enough to penetrate wood and glass. It is lightweight and comfortable to hold when you need to actually use it. Sets you back under $10.