Whether you freelance on the side or you're a full-time contractor, working out what to charge can be a challenging question. Your Rate is a webapp that asks three simple questions: how much you want to make every month, how many hours you want to work each week, and how many weeks of holiday you want to take each year. Fill those in and it calculates a suggested hourly rate.
Your Rate's calculations factors in the need to pay tax and save for retirement, though it works that out using a fairly crude basis (doubling the rate you'd require for your suggested income). You'll need to work out whether the suggested rate is actually competitive in your industry, but it's a fast way of calculating what you'll need to charge to reach a particular income goal. The webapp reminds us a little of the previously mentioned MyPrice for iOS, though that app performs slightly more detailed calculations.