Gemini III (Molly Brown)...very significant as it was the first spacecraft able to change its orbital plane and do maneuvers. Everybody else before it was pretty much just cargo.
Gemini III saw the use of the Titan II for a crewed launch. The Titan II uses hypergolic fuel for its booster engines...very dangerous as later flights found out.
Gus Grissom and John Young were at the controls. Infamous for its 'corned beef on ry
Mars Pathfinder/Sojourner July, 1997
The first US Mars rover. The Russians had one previously...but it failed and was destroyed when it landed in a violent dust storm.