Wand-Lighting Charm

Lumos is a charm that causes the tip of the caster's wand to light up. Evidently the light has some heat to it, because Alastor Moody used the light to burn paper. Its counter-spell is Nox.

Apparently, this charm is not affected by the Trace, probably because it is incredibly simple and looks like a flashlight, so you can use this spell outside Hogwarts.

Known Practitioners

 * Harry Potter
 * Ron Weasley
 * Hermione Granger
 * Quirinus Quirrell
 * Albus Dumbledore
 * Nymphadora Tonks
 * Luna Lovegood
 * Severus Snape (possibly)

Etymology
The incantation Lumos is derived from the Latin lumen meaning "light".

Behind the scenes and other versions

 * Other versions of the spell were used in the movies:
 * Lumos Solem was used by Hermione in Harry Potter and the Philosopher's Stone creates a beam of light with the intensity of the Sun.
 * Lumos Maxima was performed by Harry Potter at the Dursleys' house in Harry Potter and the Prisoner of Azkaban, and was also used in the Horcrux cave in Harry Potter and the Half-Blood Prince by Albus Dumbledore and Harry Potter. It creates an extremely large ball of light at the end of the wand, which can even be cast off to hover and light a greater area.
 * In the Harry Potter and the Prisoner of Azkaban game, Ron Weasley has a version called Lumos Duo, presumably twice as solid a beam as Lumos.

Appearances

 * Harry Potter and the Philosopher's Stone (video game)
 * Harry Potter and the Chamber of Secrets
 * Harry Potter and the Chamber of Secrets (video game)
 * Harry Potter and the Prisoner of Azkaban
 * Harry Potter and the Prisoner of Azkaban (film)
 * Harry Potter and the Prisoner of Azkaban (video game)
 * Harry Potter and the Goblet of Fire
 * Harry Potter and the Goblet of Fire (film)
 * Harry Potter and the Order of the Phoenix
 * Harry Potter and the Order of the Phoenix (film)
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)
 * Harry Potter and the Deathly Hallows
 * The Tales of Beedle the Bard

Notes and references
Lumos