The elven-forged sword of Gandalf. In The Hobbit by J.R.R. Tolkien, a company of dwarves, Bilbo Baggins the hobbit, and Gandalf were travelling to the Lonely Mountain. There, they would kill Smaug, the dragon who inhabited the mountain. On the journey there, they were waylaid by a trio of trolls named Bert, Tom, and William. Luckily, Gandalf rescued the company by tricking the trolls into arguing through the night and into the dawn. The dawn caused the trolls to turn to stone, allowing the company to be freed. In the trolls' stash, the company came upon two beautiful swords and one knife. Upon showing them to Elrond in Rivendell, the following was revealed:

Elrond knew all about runes of every kind. That day he looked at the swords they had brought from the trolls' lair, and he said: "These are not of troll-make. They are old swords, very old swords of the High Elves of the West, my kin. They were made in Gondolin for the Goblin-wars. They must have come from a dragon's hoard or goblin plunder, for dragons and goblins destroyed that city many ages ago. This, Thorin, the runes name Orcrist, the Goblin-cleaver in the ancient tongue of Gondolin; it was a famous blade. This, Gandalf, was Glamdring, Foe-Hammer that the king of Gondolin once wore. Keep them well!"

The other weapon was later named Sting in the Mirkwood. It was small enough to be used as a knife by all save Bilbo. Due to Biblo's diminutive Hobbit stature, he was able to wield this magnificent knife as a short sword. This blade played a very important role in The Two Towers. Sam Gamgee used it to wound the evil spider monster Shelob, saving the ringbearer Frodo and prolonging the quest to destroy The One Ring.

Each sword began to glow if orcs or some other evil drew near.

The sword Glamdring served Gandalf well. It is not clear whether or not he lost it after his fatefull fall off the bridge at Khazad-dûm in Moria.

Update: it is accepted that Glamdring survived Gandalf's fall. Later Gandalf tells his tale of the battle and says 'ever I hewed him', which means he injured him with the sword. He also had it later when the company rode through Bree in October of 3019. It isn't mentioned, but it is also safe to assume he still wore it when he sailed into the West.