Double boilers are great for gently melting chocolate, but we've found an even easier method that uses common kitchen equipment.
Put chopped chocolate in a small metal bowl along with any other ingredients you need to melt, such as butter. In a large frying pan over high heat, bring about an inch of water to a low boil. Take pan off the heat and set the bowl in the water. Let sit, stirring occasionally, until chocolate is melted, 5 to 10 minutes.