A fun project by Instructables user CTY1995 is great for cyclists riding city streets. It's turn signal arm bands that light up when you lift your arm.
The materials list is quite short and includes:
1. EL Panel w/ AA-powered AC Inverter 
2. Mercury Tilt Switch 
3. Velcro strips [4-6 pairs]
4. 1.5V Button Cell--LR44 / AG13 
5. Electrical Tape
6. Armband 
Lifehacker notes, "The beauty of these turn signals is that they use a mercury tilt switch and some electroluminescent (EL) panels that light up to show big bright arrows every time he lifts up the arm that corresponds with the direction he wants to move. Combined with some Velcro and armbands, the panels make a comfortable, wearable gadget that can make your bike ride much safer. Best of all, the tilt switch is sensitive enough that you don't have to start waving your arm wildly in the air to turn it on—just a slight lift will do the trick, and everyone around you will know where you're going"
It is great that you can do a slight lift of the arm, which means you may even be able to lift your arm enough to activate the signal without needing to take your hands off the handle bars. Not that that should be too big a deal -- after all you'd need to take a hand of the handlebars if you were giving a hand signal anyway.
This instructable took inspiration from one completed by gtoal, but is made for folks with a tighter budget. If you have the bucks, though, then gtoal's instructable might be more your preference.
Personally, I'm still a big fan of the turn signal jacket created by Leah Buechley with Arduino controls on the wrists.
What do you think of this design? Any ideas for improvements? Have you devised something better?