My favorite option for simple self defense (outside of a gun) is pepper spray. It is inexpensive, works well and can fend off multiple targets with less physical interaction. In other words I would rather my wife carry pepper spray than let's say a knife. I would rather she spray and run than try and engage with a bigger assailant.
If pepper spray isn't your type of thing, check out the Yoogo Self Defense Keychain ($19.99). The Yoogo keychain is compact and discreet. It is designed to work with your natural defense instincts of punching, hitting, and hooking. The Yoogo is also comfortable in your hand which is very important as you don't get to pick when someone is going to attack you. If your self defense gadget is buried in your purse or pocket it isn't going to do you much good when you need it most.
Check out the Yoogo Self Defense Keychain after the break.