Does the cut go all the way through or is it just a surface cut?

If all the way through, I wouldn't ride on it.

If not all the way through, I'd take a good look to see if any of the cords are cut. If they are, I wouldn't ride it. If not, I'd be tempted to fill up the hole with something like RTV or Shoe Goo and then use it on the rear wheel. If a tire has a blowout, I'd rather be it on the back.

One other good use for that tire is if you have a mag or fluid trainer. They tend to wear out tires quickly and this would be a good candidate for trainer use.