This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Almost never. You could say enums are singletons so that might be the only good example of use. Singletons are difficult to test, easy to missuse and tent to end up in a "Oh actually i need more than one" situation.