This is one of my favorite kitchen gadgets. The Microplane Grater/Zester is the best. Sure you could use the plain-old box cheese grater but this gives you a much better end product - finer zest, no pith, no large unwanted chunks. My favorite uses are zesting citrus, grating whole nutmeg (cheaper and better tasting than the pre-ground stuff), grating garlic, and grating parmessan cheese. Best for hard cheeses, does not work well with the semi-hard or softer cheeses.
What gadgets can't you live without?