The Ikea Lack shelf seems to be a popular choice for adding custom doors. I even did it for my own fan cooled media unit. If you want a different look from sliding doors though, tambour doors are another option.
Tambour doors are those sliding doors made up of thin slats of wood. These doors slide away into the cabinet when opened and were commonly used in sideboards from the 1950's. Ikeahacker Thomas reminds us that this could be done easily with any cabinet with an open front. Lee Valley has tambour door kits which would be perfect for a DIY project, but unfortunately the instructions from Ikea Hacker are in German so you are on your own there unless you happen to be from Deutschland...or know someone who is.