Viking 1
Viking 1 was the first of two spacecraft (along with "Viking 2") sent to Mars as part of NASA's Viking program. On July 20, 1976, it became the second spacecraft to soft-land on Mars, and the first soft lander to successfully perform its mission. (The first spacecraft to soft-land on Mars was the Soviet Union's Mars 3 on December 2, 1971, which stopped transmitting after 14.5 seconds.) "Viking 1" held the record for the longest Mars surface mission of days (over 6¼ years) or Martian solar days, until that record was broken by "Opportunity" on May 19, 2010.
Mars Cube One
Mars Cube One (or MarCO) was a Mars flyby mission launched on 5 May 2018 alongside NASA's "InSight" Mars lander mission. It consisted of two nanospacecraft, MarCO-A and MarCO-B, that provided a real-time communications link to Earth for "InSight" during its entry, descent, and landing (EDL) on 26 November 2018 - when "InSight" was out of line of sight from the Earth. Both spacecraft were 6U CubeSats, and the mission was a test of new miniaturized communications and navigation technologies. These were the first CubeSats designed to operate beyond Earth orbit, and aside from telecommunications they also tested CubeSats' endurance in deep space.