Larry Niven (author of the Ringworld series of books) was once approached by Del Rey Books to write a Halo novel, but turned them down due to his lack of familiarity. Microsoft had even sent him an Xbox and a copy of Halo: Combat Evolved (among other games) to try to convince him.
There was a Halo game partially developed for the Nintendo DS, but it was ultimately canceled. The game had been developed to a point of being playable.
After Apple and Google created their own Intelligent Personal Assistant, "Siri" and "Google Now", Microsoft made their own system under the name Cortana, based on the Artificial Intelligence character from the Halo series. Cortana's voice actress, Jen Taylor, returns to voice the system, too.
During the development of Halo 3 and Halo Wars, Microsoft gave the rights to different companies to make a Halo MMO, as well as and a handheld Halo game. Both of these were never finished.