My favorite is a red, blue, and white Giro Atmos, and I have a Pneumo in similar colors as a backup. For mountain biking and commuting, I have an acid green & white Giro Xen, tricked out with lots of reflective tape, that is super-comfortable. I like Giro helmets, initially a little bit for the "Lance factor," but I've also gotten six years of good fit, comfort, and service from them, and they've taken good care of me at least on three occasions when I was pushing the boundaries of my skills.

As Ama noted, you really can't buy a new helmet these days that doesn't meet at least one (or all) of the ANSI, Snell, or CPSC safety standards. So, the $20 helmet from Wal-Mart or Target will protect your noggin as well as the newest $200+ Giro Ionos or whatever. What you're paying extra for is style, and maybe a little more comfort.