The great lads at Instructables have put together a USB charger that relies on your end product of expiration, CO2, to get your gadgets charged up. Specifically, it functions on breath power rather than CO2 itself. By attaching to your weedy chest, a generator is driven to produce energy by a system of pulleys that expand and contract as you breathe. This allows the energy produced to be passed on to your MP3 player so you have tunes for the last 3 minutes of your journey home.
The amount of energy produced depends on the amount of breathing going on, so wearing this during sex will ensure no power is produced—lets face it, if you are having sex the only excitement will be 2 seconds long and involve no heavy breathing. Having it hooked up when you go for a jog could be useful, if it is robust enough to withstand the impact. If you fancy getting your hands on one, you shall have to get your DIY skills flexing, as the only way to acquire on one is to make one yourself! Hit the link for the guide. [Instructables via Treehugger]