Panzi's addon will load bitmaps into Blender and converts them into simple voxel models. Pretty cool!
Add-on that imports pixle art images as colored cubes. Pixels with 100% alpha are ignored.
This add-on imports pixel art images as colored 1x1 cubes. Pixels with 100% alpha are ignored. All pixels of the same color will share one material. The material can use nodes (blender internal or cycles nodes, depending on what renderer is used while importing). The names of the created cubes and materials can be controlled through format strings.
The imported pixels are all parented to an empty object so they can be easily transformed as a whole. The cubes have a size of 1x1 so they can be easily moved around pixel-by-pixel while holding ctrl.