I’m a big fan of the serial comma, and the Chicago Manual of Style now “strongly recommends this widely practiced usage, blessed by Fowler and other authorities…, since it prevents ambiguity.” Here’s an example from the Times that shows what can happen without the serial comma: “By train, plane and sedan chair, Peter Ustinov retraces a journey made by Mark Twain a century ago. The highlights of his global tour include encounters with Nelson Mandela, an 800-year-old demigod and a dildo collector.” Perhaps the most famous example of why the serial comma should be used is this apocryphal book dedication: “To my parents, Ayn Rand and God.” (For the origins of these two examples, see this 2006 Language Log post and this 2003 Language Hat post.)