The ultimate in dork bikewear. They look dumb as hell, and yet work like black magic. Basically, it's two long socks with no toe closure, an unattached sleeve of moisture wicking fabric. One simply places hand though hole, rolls warmer up arm sock puppet style, and viola! You are now warm of arm and dumb of looks.
This begs the question, "Why not just wear a long sleeved jersey, or a jacket?" Well, it all comes down to the arcane art of Thermostatting. A jacket is awful hot on a bike, it needs to be very cold out to merit wearing one, as once you get started, you are your own personal furnace. Plus, the sleeves will only roll up so high. The same goes with the long-sleeved jersey. Once you're too hot, there's nothing you can do but suck it up and sweat it out.
But the humble arm warmer...once you're hot, just pull it off, roll it up and stick it in your pocket. Your arms can now act as radiators, and you don't overheat. Vice versa, if you're out riding and it starts to get cool on you, pull them out and roll them on like a giant arm condom. They really work!
And, at the superbowl, Britney Spears wore a sock on her arm, while she performed with Aerosmith. Now that looked stupid (or stupid-cool)