While glow-in-the-dark paint is not necesarily a useful invention, it is definitely a testament to the Chinese persistence in alchemy.  They had no particular reason to attempt to invent anything that phosphoresced, or glowed in the absence of light; they just stumbled on it in their pursuit of immortality through alchemy.  It is not an entirely useless invention, however.  If used on sections of the Great Wall when it was under attack, assuming the attackers could be held off until nightfall, it would probably cause invaders to fall on their knees and bow down, run away screaming as fast as they could, or collectively scream "Witchcraft!!!!" and start hanging people.  Either way, they would be effectively conquered.

Date of Invention:
At or before 1000 AD

Inventor: 
Unknown

Importance to Chinese civilization:
Demonstrated their alchemic stubbornness; repelled superstitious invaders.

Importance to other civilizations:
Entertained many young children