Step up and get your own "dynamo hum"...(FZ). All jokes aside, its a simple dynamo light mounted to a Planet Bike light bracket. On about 6 cranks I get 20 to 25 minutes of light.
There is a 3.6 volt NiMH rechargeable battery that comes in the light. SO, by free I refer to your out of pocket cost on batteries.

Step 1: Material

1. dynamo light....Harbor Freight, 2 for $8.
2. quick release light strap...most have a screw to change the mounting base, this is a Planet Bike
3. super glue
4. fingernail polish remover........or acetone

Step 2: Light Base

Remove the screw and discard the top thin portion.

Step 3: Level Base

Use a knife to take any high points off of the base.

Step 4: Break Light Down

Take the 4 screws out of light.
Be cautious not to let any gears or gear shafts to fall out.

Step 5: "drill" Hole for Screw

Using a hole punch I twist and push a hole through the plastic, working from both sides.

Step 6: Prep for Glue

Using the fingernail polish remover, I clean both surfaces.

Step 7: Glue

1. I put glue on the base only.
2. Screw light cover to base.

Step 8: Put It Back Together

Once glue is dry, put your 4 screws back in and your done.

Step 9: Get Rolling and Give It a Crank!

The law (here in Arizona) says you need a forward facing light visible from 200 yards. This covers that and I only need a rear red reflector. So, I'm now battery free!