Here's a nice hack to have. And considering if you like night biking, then this is a must, for safety! You can turn any jacket or sweatshirt into a signaling sort of jacket by adding turn signal LEDs on it.
Instructables user leahbuechley (Dr. Leah Buechley) used this device to test out her Lilypad - an Arduino designed for wearables and e-textiles. It powers a set of left and right turn LEDs as well as power and switch indicator LEDs on the jacket.
You'll need a LilyPad Arduino board, an FTDI connector, power supply, LED lights, 2 push button switches, conductive thread and general hand sewing gear like fabric glue, needles, markers and scissors.
Sew the LilyPad, FTDI and power supply and test the circuits before adding the lights and sealing the electronics. Then program the LilyPad, plug in the battery pack and you're good to go.