Here is a work in progress. An attempt to list all the robotic spacecraft and vehicles exploring space for us.
Sputnik 1 was the first artificial satellite to orbit Earth. Launched on the 4th of October 1957 by the Soviet Union (USSR).
Explorer 1 was launched by the USA on the 1st of February 1958 to study high energy particles, subsequently discovering the Van Allen radiation belt surrounding Earth.
The Explorers program uses artificial satellites for small to medium sized missions with a science focus.
More information here.
Luna 1 was a Soviet Union (USSR) spacecraft also known as Mechta, designed to study interplanetary matter, cosmic radiation and the magnetic fields of the Moon and Earth, before impacting the moons surface. Luna 1 unfortunately missed the moon and to this day remains in orbit around the Sun, between the orbits of Earth and Mars.
The Soviet Luna program successfully launched 24 robotic satellites between 1959 and 1976. More information here.