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?