This is possibly the most famous sword in the books and films of Lord of the Rings. Sting was an Elvish long knife made in Gondolin during the First Age. It is unique for being feared by the spiders of Mirkwood.
Sting was an ancient blade made by Elvish weapon-smiths in Gondolin. When ever orcs (goblins) come near it glows blue, warning the wearer. The blade was carried by Bilbo in The Hobbit after he found it in a Troll-hoard. Sting was found alongside Glamdring and Orcrist. Though just a knife by the standard of elves, it made a perfect short sword for a Hobbit, although it was still rather small, as Bilbo initially chose to wear it "inside his breeches" and was still able to travel and even run without any apparent inconvenience.
Just before his nephew left on his quest to Mordor, Bilbo gave Sting to Frodo. Sam then took the weapon from his (seemingly dead) master and used it to good effect against Shelob on the borders of Mordor. After the defeat of Sauron at the end of the Third Age, Frodo entrusted Sting to Sam and it became an heirloom of the Gamgee family.
Gollum, who disliked anything made by the elves, was afraid of Sting. This fear aided Bilbo when he confronted Gollum in a cave at the base of the Misty Mountains in The Hobbit . It also helped Frodo and Sam subdue Gollum in The Lord of the Rings. Orcs also had an instinctive fear of these weapons and hated any who carried them.
As is fitting for a blade of Gondolin, Sting could easily cut through the webs of the offspring of Ungoliant, including the evil Shelob and the spiders of Mirkwood, who christened the weapon Sting after many of them had been "stung" by Bilbo with it.
Sting was like Glamdring and Orcrist in that "being the work of Elvish smiths in the Elder Days these swords shone with a cold light, if any Orcs were near at hand." But only Sting was definitively described as glowing blue, or glittering with blue flame at its edges.