Remember what the law says in your area about use and intended use, too. For instance, the people that suggest wasp spray are playing with fire as the can says it is a crime to misuse it. If you can get pepper spray, get Fox Labs or Phantom - I can personally attest to the effectiveness of Phantom.
A knife can be useful, but get some training, if you can - knife fighting can be an art form.
The civilian Taser is the best - fire, drop and run - it cycles for 30 seconds, long enough to get out of the area.