Interesting question. A couple of ideas come to mind. You may argue with my points, but I do believe there are things that exist in negative quantities that are real, physical things.
For instance, what about positive and negative charge? Negative charge is not imaginary, but it is a physical reality measured in negative amounts. The reason you can be sure of this is that if you take two particles of equal but opposite charges, then the net charge is neutral (0). Thus, the negative charge subtracted from the positive charge, making the negative charge a negative amount or quantity.
Also, in the study of wave behavior, waves often have imaginary components. Waves are important in engineering and physics (light is a wave...).
I'm sure there are other uses. I searched google and found a link back to yahoo answers from someone who asked a similar question. I posted the link in my sources.
Hope this helps.