I recently came across a blog post at Get Rich Slowly that was about prepaying your mortgage. After reading it I am convinced that for some people prepaying the mortgage is a good investment.
The basic principle of the plan is to make two payments each month. One payment to go towards the mortgage, including all the interest, taxes, fees, and principle, and another payment to go towards the principle.
You had better read more.
I suppose this method could be applied to any loan type situation. Maybe student loans...