some quick cash. Or, take some classics from your personal library and sell them at a local second-hand bookstore. 3. Search circulating coinage Susan Headley, the guide to coins on About.com, is a lifetime coin collector who has been boosting her income by searching through circulating coinage for the past six years. In 2008, she made about $2,500 and so far in 2009, she has earned approximately $500 from coins she's found. People who search circulating coinage successfully for a side income do so in very large numbers, she says. They buy rolls of coins from banks, typically in whole boxes, and sort through it to find stuff that just does n't belong, Headley says. Half dollars, for example, were no longer made from 90 percent silver after 1965, but they still had 40 percent silver in them until 1970; either of these turn a nice profit. Presidential dollar errors can be worth $50 to $5,000 each; uncirculated state quarters can sell from $10 to $50 per roll; and rare error coins can value up to $35,000. 4. Start a business Turn your hobby, skills or expertise into a part-time business. Sites like Jobvana can help you do so by providing you with free tools to market your services and offer specialized skills to those looking for help. Peter Olson says he built a profile in September 2008 offering to teach guitar lessons. He has since gained two students, earning about $240