If you are showing, Vetrolin Shine. Saved my butt when I pulled the cooler off my lease horse at the finals last October only to discover he was covered in dust. -.- Not fun.
Plus, a good rubber-coated hoof-pick. This could be considered "frivolous," but considering how often I pick Sasha's hooves, having a hoof pick with a soft handle is a must. Duct tape works well for padding up the handle, too. But a good curry comb [I agree with a Jelly Scrubber,] a hard brush, a soft brush, and possibly a wide-toothed comb for manes+tails are what you should be looking for. Natural-bristled [not nylon] brushes work the best, but can be expensive. You may also choose to have a rag [an old dish cloth works well] to rub the horse with.
Have fun with your lease!
Oh cry me a river, build a bridge, and get over it.