I was browsing everyone's favourite auction site earlier, and came across a soldering iron designed to run off a USB port.
The tip (if you can call it that) looks terrible, the temperature regulation is no doubt non existent, the idea is pointless... but I can't help but wonder if it would even get hot enough to melt solder (ignoring any components or traces for a moment).
USB allows up to 2.5W, two ports may allow for 5W best case - which sounds very low to heat up that much exposed metal. They also show it with a 9v battery adaptor - I'd be very surprised if they could get close to 5W out of a standard PP3 battery, although the voltage drop probably means they're safe without having to design it for dual 5v/9v use...
I think it's the worst soldering iron I've come across - can anyone beat it?
Andy