OC
Pepper Spray
Liquid Hell
This less than lethal tool is either well-touted or much maligned, depending on what self-defense instructor you speak with.
To be fair, much like guns, knives, and other tools, not all OC Sprays are created equally... and like most other tool choices, you USUALLY get what you pay for.
There are a few tricks you can use to make a deployment of OC much more successful.
The spark to this is my very successful deployment of OC Spray to halt (near instantly might I add) an assault in progress over the weekend.