I carry a Monadnock "Persuader" Kubaton as my keychain. I also can also recommend the Mini-Maglite. The Mini-Mag is a great choice for children in school or anyone who works in a PC environment, as a true Kubaton will likely be considered a weapon by the sheeple and their keepers.
The man who trained me to use it related the following story during the course. Paraphrase: His 16 year old grandaughter was being harrased by a boy in her high school. This large football player manchild would approach her in the hallway from behind and grab her a$$. She came to granddad (arguably the most experiencied self-defense instructor in the state) for help. He gave her a mini-maglite and taught her how to use it. Next day "Mongo" walks up behind her for a feel, she turns executes the technique, and puts sad sack on his back. He never bothered her again. In fact his shock and embarrasment at being bested by a little girl was so complete, he would back away and hunch down in a submissive pose whenever he saw her from then on.
The other tool I carry is called a "Lapu Lapu Corto". A smaller version designed for women is called the "Tusok". It is a small folding pocket knife with a ramp on the back of the blade and a special handle. It is designed to be used in the blade folded and closed position as an impact or pain compliance device. If the less than lethal technique fails, then the knife can be opened and used like any other knife. I learned to use it at the same school as the Kubaton, but from a different instructor.
The key to either is training. Having the tool is nice. Having the skill and confidence to use it is essential.