There have been several King's Quest fangames both original and retellings/remakes of the original games that have been released by various developers.
Interactive Fantasies created two of the earliest KQ fan adventures were created while Sierra's was still producing games in the King's Quest Series. In 1997, they created two overhead adventure action-RPG games set after KQ7 using Epic MegaGames' ZZT software, King's Quest ZZT and King's Quest ZZT 2. In an idea predating 1998's Mask of Eternity, the game includes weapons and enemies to fight (in a similar style to Ys series by bumping into the enemies, or manually firing arrows), and even includes a few side-scrolling platformer arcade sequences.
AGD Interactive (previously Tierra Entertainment) has released three King's Quest games so far. First, they created a more or less straight remake of King's Quest: Quest for the Crown, with advanced VGA-style graphics and a voice soundtrack, King's Quest I VGA. They moved onto create a retelling of King's Quest II: Romancing the Throne, creating an all new story inspired by the original game. In February 2011, they released King's Quest III Redux, an enhanced retelling of KQ3 (continuing ideas that first appeared in Romancing the Stones). It was not a straight remake as their KQI remake, nor was it a full overhaul like their KQII remake had been; in fact, many of the elements of the original KQIII story were left intact, but details were altered to create a more challenging experience, and the story was tweaked to incorporate elements of the KQII rewrite as progressing into KQIII.
Infamous Adventures released a remake of King's Quest III: To Heir Is Human (on June 19, 2006). The release includes Video Graphics Array-style graphics and an advanced music and voice soundtrack for the game. It offers several new and extended cut scenes, a few tweaks to locations, a new ending, and a streamlined spell casting system.