Minecraft now uses resource packs instead of texture packs. When it used texture packs a program call MTE (Minecraft Texture Editor) allowed you to open two different texture packs and move textures of individual blocks from one pack to another in a drag-n-drop fashion. It also displayed a small 3-D version of the item.
It doesn't look like it will be updated by the creator to the new resource pack format. While it would be nice to have a 3-D view of blocks, it's more important to me to be able to drag-n-drop textures from one pack to another.
Anyone willing to accept this challenge?