Two small probes named Amundsen and Scott, part of NASA's New Millennium Program. Roughly the size of basketballs, they were dropped from the doomed Mars Polar Lander. They were supposed to examine Mars' south polar subsoil for signs of water. Expected to land on Dec 3, 1999 they never did phone home and they were given up for lost on Dec 7, 1999.
If they had worked, the 5.3-lb probes would have impacted the Martian surface, shattering their protective areoshells. Each would then have pushed probes about 2 feet deep into the soil. Their mision was to determine if ice was present below the Martian surface, and to characterize the thermal properties of the subsurface soil. Data would have been relayed back to Earth via Mars Global Surveyor.
Each carried ten first-of-a-kind instruments and systems, and there was benefit to developing and deploying them, even if they did fizzle at the finale.