In Norse mythology, Eitri (or Sindri) is a dwarf and the brother of Brokkr.
According to Skáldskaparmál, when Loki had Sif's hair, Freyr's ship Skidbladnir and Odin's spear Gungnir fashioned by the Sons of Ivaldi, he bet his own head with Brokkr that Eitri would not have been able to make items that matched the craftsmanship of those mentioned above.
Eitri began working in his furnace while his brother was working the bellows, but a fly (sometimes thought to be Loki himself) began stinging Brokkr, trying to stop him and spoil the items.
Eitri succeeded in making the golden boar Gullinbursti, the golden ring Draupnir, and the hammer Mjöllnir that made his brother win the bet, even if its handle was shorter than it should have been.
In other media
Eitri appears in Jim Butcher's the Dresden Files: Cold Days as Mr Eitri.
In Rick Riordan's Magnus Chase and the Gods of Asgard series, a fictional son of Eitri, Eitri Junior, appears as a rival to one of the main characters, Blitzen, in the first book, The Sword of Summer. Eitri himself is mentioned as the creator of the original rope that bound Fenrir, Gleipnir.
Eitri occasionally appears as a supporting character in the Marvel Comics, including the "New Mutants Special" released in 1985. He becomes an ally of the titular heroes when a member of their team saves his beloved daughter. He is also a supporting character in the Ultimate Spider-Man episode "Field Trip", where he forges magical weapons for Spider-Man's team to take down Loki in support of Thor.
Eitri is mentioned as a Khûzdul mercenary in the Hârn supplement "Friends, Foes, and Followers #8"