AKA: Mobile Equipment Transporter. Status: Operational 1971.
The MET would extend lunar surface activities to a greater distance from the lunar module prior to availability of the Apollo Lunar Rover vehicle. It was planned to use the MET on Apollo 14 and 15. But the Apollo 15 H class mission was cancelled. So the MET was only used on Apollo 14.
It's a common misconception that the MET was going to be carried on Apollo 13. It was originally to have flown on the Apollo 14, 15,and 16 in the original mission numbering scheme - the two H-class missions and the first J-class mission. Then Apollo 17-20 would get the rover. When three Apollo missions were cancelled, the second H-class missions was deleted, the missions were renumbered, and the rover was included in the first J-class mission. In the renumbered scheme only Apollo 14 got the MET and the rover was used on the renumbered Apollos 15, 16, and 17.
NASA was considering incorporation of a mobile equipment transporter on LM-8, LM-9, and LM-10, to help with problems such as the Apollo 12 astronauts had in carrying hand tools, sample boxes and bags, a stereo camera, and other equipment on the lunar surface. The MET also could extend lunar surface activities to a greater distance from the lunar module. A prototype MET and training hardware were being fabricated and were expected to be available in late December.