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Almost any general-purpose twist bit will do a decent job of drilling holes in metal. In fact, most drill bits for metal are manufactured to drill through a variety of materials, including wood and plastic. The least expensive twist bits are made of high-speed steel (HSS), and these basic bits are just fine for most metal-drilling tasks. If you'll be drilling a lot of holes, or need to drill through hard, abrasive metals like stainless steel or cast iron, spend a couple bucks more for black oxide or a cobalt steel drill bit for metal. These bits will bore more holes before becoming dull. Some bits also have a special coating called titanium nitride (TiN), which manufacturers claim helps resist heat and friction better, making these bits last up to six times longer than standard high-speed steel bits.