The original multi-hotends from E3D!
Dual Extrusion brings the abilities of your printer to a whole new level; providing a way to print items in multiple colours, multiple materials, support materials, all giving you unprecedented freedom in what you can print. Multiple colour prints will be the envy of your friends and allow you to convey more visual information in prototypes.
Multi-material prints allow you to combine distinctly different material properties in a single print, imagine rigid Carbon-Fibre-PLA wheel hubs with printed in place NinjaFlex tyres or a Nylon bushing.
Support materials allow you to ignore the age old problems of overhangs or picking off tedious-same material supports by using soluble support materials, or materials that break away cleanly from your prints.