Omakes

when used outside of Japan, it usually refers to specials and outtakes in various forms of anime and manga-related media.

Manga Omakes
D.Gray-man Manga omakes include character profiles, questions answered by Hoshino/members of the D.Gray-man cast, illustrations of various characters' rooms, etc.

List of Manga Omakes

 * Character Profiles (Chapters 1, 3, 5, 6, 8, 9, 10, 23, 24, 27, 28, 43, 44, 66, 67, 139, 140, 141, 142, 143, 144, 145, 146, 147, 189, 190, 191)
 * Discussion Room
 * Komui's Discussion Room
 * Komui's Discussion Room Special Edition: Noah's Ark
 * D.Gray-man Investigation Series (?)
 * Character Popularity Poll (Chapters 63, 64)
 * Character Bedroom Sketches (Chapters 73, 74, 80, 81, 82, 83, 84)
 * The People of D.Gray House
 * Pointless D.Gray Stories
 * D.Gray Theater

Anime Omakes

 * D.Gray-Omake Mini Theatre

Fanbook Omakes
Several official compilation fanbooks/illustration collections have been printed, with the Gray Ark and CharaGray fanbooks including detailed character biographies, story arc outlines, and collections of character polls, fan-submitted artwork and official Q&As.
 * D.Gray-man Official Fan Book Gray Ark
 * D.Gray-man Illustrations Noche
 * CharaGray
 * D.Gray-man Official Fan Book Gray Ark
 * D.Gray-man - Artbook - Gray Walker