Gollum: Before he was corrupted by the ring, Gollum lived near the River Anduin where he went fishing with his friend Déagol. He was a Stoor, and at that time, he was known as Sméagol.
One day in TA 2463, Déagol saw something at the bottom of the lake, and he went to get it. When was sitting on the bank with it, Sméagol asked if he could have it for his birthday. Déagol said he had already given him a present and that he wanted the gold ring. Sméagol then strangled Déagol and took the One Ring.
He kept the ring secret, and used it to help him find out secrets among his tribe. Eventually he started stealing and was shunned by his family. He started muttering, and gurgling in his throat, and it was then people started to call him Gollum. His grandmother exiled him from the family to keep the peace.
In TA 2470, he found his way to the Misty Mountains. There, the ring continued to corrupt him, and he became was a skinny, crawling creature, bearing almost no resemblance to a hobbit. He lived on an island in the middle of an underground lake where he lived off fish and the odd orc that got lost in the tunnels.
In 2941, he met Bilbo Baggins, who had been separated from his Dwarven companions. Gollum challenged him to a game of riddles, which Bilbo won. Gollum realised he had been tricked when he found that his Precious (his name for the One Ring) was gone.
Gollum later went after the Ring, and he was captured coming back from Mordor by a Ranger named Strider. Strider brought Gollum to the Elves of Mirkwood where they kept him until he escaped with the help of Sauron's minions.
He was next seen in Lothlórien, and from there he followed the Fellowship until they parted. Then he followed Frodo and Sam. They noticed they were being followed on their road to Mordor, and captured Gollum, ordering him be their guide to Mount Doom. He agreed, after swearing on the Precious.
Gollum brought them to the stairs of Cirith Ungol, and led them up into the dark cave. Then they were attacked and Frodo was stung by Shelob. Gollum tried to kill Sam himself, but was overpowered.
Gollum caught up with Frodo and Sam again at the Cracks of Doom where Frodo
tried to cast the Ring into the fire. Gollum ran up to Frodo and bit off the
finger that the Ring was on. He danced with the joy of getting his Precious
back, but tumbled backwards and fell into the fiery chasm.