The Fall of Gil-galad
Little is known of the lay itself. According to Aragorn, it was originally written in 'an ancient tongue' (presumably Quenya), but Bilbo Baggins later translated it into the Common Tongue, and taught it to Samwise Gamgee in his younger days. It seems to have told the story of the War of the Last Alliance, at least to the point where Gil-galad aided in the overthrow of Sauron, and was himself slain. Its contents, though, are largely unknown to us: Sam confirms that it was a long poem, but apart from three introductory stanzas, the text of the lay is lost. Aragorn does say that he knows more of the text, as does Frodo, but he refuses to share it at the time he discusses it with the hobbits.
Sam's verses of the song are given in Book 1, Chapter XI of The Lord of the Rings (A Knife in the Dark).
A musical version of those three verses was recorded by the Tolkien Ensemble on their album A Night in Rivendell.