I was removing the top to my terrarium, and I dropped it on my leopard geckos tail. He didn't drop his tail but he has an open wound. What should I do?
I never owned a gecko, but you should take a careful look or take it to a vet.
I know that their tails have massive regeneration and might even fall off with stress or something.
Try to treat the wound and remove any substrate or dirt that has stuck to the wound. Apply disinfectant (see What antiseptic is safe to use on reptiles) to the stump using a cotton bud.
http://www.moonvalleyreptiles.com/reptile-resources/reptile-health-and-nutrition/wound-care .. is an excellent link that I found about reptile wound care. I myself own two African dwarf frogs, but I understand they're not quite the same as a non-aquatic reptile, heh.
All wounds, regardless of species, should be kept clean to prevent infection. If the wound is deep, apply pressure to staunch bleeding and contact your vet as soon as possible, as your scale baby may require stitches and/or oral antibiotics. Let me know how it goes!