Topping the list of useful Mars technologies is a tech so powerful that it provides a primary motive for sending human explorers to Mars: Telerobotics. The awesome power of humans and machines working together demands an equally awesome challenge, the exploration and conquest of the vast untamed wilderness of Mars.
Existing NASA rovers like Curiosity or Opportunity have accomplished significant scientific contributions, yet these machines are mere toys compared to the next generation of machines for exploring Mars once humans are close enough to control them.
Think about how humans explore and develop the Earth. We rely upon human-guided automobiles or airplanes for transportation, human-guided construction equipment for building grandiose high-rises, human-guided biology equipment in the laboratory, etc etc etc. Placing humans "in the loop" leads to exponential growth in capability as well as the ability to use well-proven equipment and techniques from Earth.
Yet Mars poses unique challenges because we must keep these valuable human control geniuses safe. Therefore, the primary use for telerobotics in an early Mars settlement will be to reduce risk. Humans will minimize their excursions into the dangerous surface environment once they have the option of controlling a human-shaped, hardened robot that relays tactile sensations back to the user.
In this area more than any other, NASA leads the way in technology research applicable to Mars settlement. The goal of NASA's highly successful Robonaut (pictured) program is to explore a planetary surface using teleoperated robots. These robots can use human-designed tools. Through sensory feedback, a human controller can use those tools naturally. Robonaut prototypes also have a limited ability to learn tasks, so a human controller can let the machine take over autonomously once it knows how to do a repetitive task.
Other useful telerobots may include roaming cameras, rovers, diggers, balloons, and hoppers. As more machines come to Mars and help unlock its vast potential, networks of machines will work together to accomplish even more. These networks will require significant navigation and control information. Perhaps someday soon, Mars will have its own constellation of GPS satellites?
The primary long-term concern with telerobotics is wear and tear. However, as was noted earlier in the list, 3D printing offers the ability to create new parts as old ones wear out. Robot maintenance will be a primary task of the human engineers maintaining the settlement.