I have the Lumos helmet that Ryan mentions, and have also had a couple of "bike turn signal" systems. My wife, who usually FOLLOWS me, says NONE of those have much visibility, and that arm signalling is still MUCH BETTER. Remember that to most car drivers, bicycles are INVISIBLE things, so MORE IS BETTER with any kind of signals. I am particularly intrigued by the recent turn signal gloves I've seen advertised, but I won't spend the money until I am CERTAIN they would actually accomplish something.
At night, ANYTHING may work, but in daylight, NOTHING seems to work.