The Bicycle Helmet Safety Institute has published the annual review of helmets and helmet technology.
Summary: Our review finds no radical safety improvement this year that would compel you to replace your current helmet, although new and promising technology is finally coming to the marketplace. Almost all of the helmets described below meet standards and offer good if not excellent protection. We have tested a sample of cheap and expensive helmets and found no real performance differences by price. We recommend looking for a helmet that fits you well and has a rounded, smooth exterior with no major snag points.
Read more about trends, new models, and more at http://www.helmets.org/helmet14.htm.