The file
A plush worm in a green sleeper whose face lit up when you hugged it. It was a night-light you could hold, and for a lot of kids born in the late '70s it was the first toy they remember.
The original's soft yellow glow came from a real bulb and real batteries, and it faded when they ran down, which parents learned about at 3 a.m. There was a cartoon; nobody remembers it. They remember the glow.
Sources: Wikipedia — Glo Worm