A circuit that can solder itself and then be used to solder other boards!
The concept is simple, use a multi-layer board, and design one of the layer as a resistive copper thermal track. Powering this heater to around 165°C can melt low-temperature solder-paste, while still keeping the board at a safe temperature (for less than 5min).
Soldering RepRap was also achieved by having a reflow controller circuit. This means that once the PCB gets soldered, it can also reflow other circuits like it, that are using the same self-soldering concept.