Alien

While there is some evidence of the existence of Extra-Terrestial life forms, it is very scarce and there is no evidence of there having ever been actual contact with these creatures.

Both the Brotherhood of Steel and the Shi have come into contact with bodies of alien life forms or their spaceships, as did the pre-War United States government, which kept its alien specimens mostly in Area 51.

Contrary to popular belief, the creatures known as "aliens" or "wanamingos" are not of extra-terrestial origin, but were created through genetic engineering.

Crashed ships
Two alien ships are known to have crashed in the United States:
 * Alien Ship in California
 * Recon Craft Theta in Capital Wasteland

Alien technology

 * Alien Blaster - a weapon of alien origin
 * Firelance - a stronger variant of the former
 * Alien Power Cell - the power source for the Alien Blaster
 * The Skynet computer was created based partly on alien technology (According to Skynet's own records anyway, which if cross referenced with other sources appears to be incorrect.)

Appearances
Crashed alien ships appear in Fallout and Fallout 3. Alien Blasters also appear in Fallout 2.