
[Che-Wei Wang] has a few more projects up his sleeve aside from his pneumatic power suit. This is a bit artsy, but he created an ultraviolet ink dropper to leave a trail of invisible ink wherever he rides his bike. Just think of it as a modernized version of [MacGyver]'s paint can under the bad guys car tracking system.
http://www.hackaday.com/2008/07/25/uv-drip-bicycle-tracker/