Cross-bows are only known to be have been mentioned once in the stories about Arda, as a weapon devised by the Númenóreans. While used in particular to shoot predatory birds, cross-bows appear to also have found other use in Númenor.
Portrayal in adaptations
1982-97: Middle-earth Role Playing:
- As part of the roleplaying game's extrapolation of Middle-earth's aspects, crossbows occur as a rarer form of archery weapon. They are used most commonly by the dwarves, who are implied to be their original inventors. They can be sometimes found in the possession of some half-elves and half-trolls as well. Hobbits also occassionally use them, specifically smaller light crossbows. These are referred to as "bracebows". 
- In the film, the Uruk-hai use fairly large crossbows at Helm's Deep, presumably designed by Saruman. The Uruk crossbowmen are featured relatively little in scenes during the siege, mainly at the beginning and while trying to conquer the first gatehouse of the fortress, shooting at the defenders through gaps in the damaged gate. They use their crossbows with ease and deadly accuracy. Though the crossbows are reloaded somewhat slower than a bow, they feature an integrated spanning lever, which allows for faster spanning of the bowstring and makes additional spanning tools unnecessary. In the behind the scenes documentaries of the film's Extended Edition, concept artist John Howe and production designer Richard Taylor briefly discuss and demonstrate the Uruk crossbow props, made by Weta Workshop. Howe reveals that the inspiration for the crossbow came from an early 16th century design, found in the Codex Löffelholz manuscript from Germany.   
- Uruk crossbowmen are portrayed as a deadly fighting unit.
2007: The Lord of the Rings Online:
- Crossbows are available to player characters as handheld ranged weapons. They are less common than bows. Some player character classes, such as the Champion, are not allowed to use the weapon. 
- A steel dwarven crossbow is one of the ranged weapons available to the (non-canon) dwarf player character Farin.
- J.R.R. Tolkien, Carl F. Hostetter (ed.), The Nature of Middle-earth, "Part Three. The World, its Lands, and its Inhabitants: XIII. Of the Land and Beasts of Númenor", p. 340
- S. Coleman Charlton (1993), Middle-earth Role Playing (2nd edition, hardcover) (#2000)
- S. Coleman Charlton (1993), Middle-earth Role Playing (2nd edition, softcover) (#2001)
- Animated image of Richard Taylor demonstrating the functional Uruk crossbow prop, Weta Workshop featurette, The Lord of the Rings: The Two Towers Extended Edition
- Codex Löffelholz in .pdf format at the Jagellonian Bibliotheque - historical design that inspired the film crossbows by Howe and Weta Workshop
- Video demonstrating a professionally reconstructed Codex Löffelholz crossbow - the rare historical design that inspired the film's Uruk crossbows
- Crossbow Items The Lord of the Rings Online Wiki