The Oxford comma--you knew I'd have to bring it up eventually, right? Without it there is often room for ambiguity--sometimes adding unintended humor. For example, “A notorious gambler, Charlie Sheen owed money to his ex-wives, Billy Bob Thornton and Hugh Grant.” In this sentence, a simple comma would prevent Billy Bob and Hugh from being noted as Charlie Sheen's ex-wives! In journalism the comma is often dropped as a way of saving printing space. However, for all other types of writing the Chicago Manual of Style recommends its usage.