The Electronic Thumb
Legend has it that Douglas Adams thought up the idea for "The Hitchhiker's Guide to the Galaxy" while lying in a field, recovering from drinking a bit too much during a trip through Europe. It wasn't uncommon for students and other travelers to hitch a lift now and then as they crisscrossed the continent, visiting new cities and phoning home for more money. What if, thought Adams, the same thing happened on a universal scale? He constructed a tale of a befuddled human named Arthur Dent and an alien in disguise with the vehicular moniker of Ford Prefect and the rest is history.
But how do you hitch a ride with an alien? You use an Electronic Thumb. Adams explains that there is a communications channel called the sub-ether network. The electronic thumb taps into this network and signals nearby spaceships to hitch a lift. Adams wrote multiple versions of his story and no two are exactly alike. It's not entirely clear that the thumb requires the spaceship's driver to give permission before the hitchhikers zap aboard using a matter transference beam.
It's true that for the electronic thumb to really be useful we'd need to have some aliens flying around first. But even if there aren't any bug-eyed monsters in the nearby galaxies, it would still make a lovely paperweight.