Did a search on this effect and found one thread where it was explained how to do it inside, but it wasn't about the actual box effect, it was a request thread. Therefore I've made a guide on how to do itI also posted this on another thread, but this will be easier to follow and easier to find.
Examples (look at the 'box' effect):
This is how you do it:
First click Filter, then Pixelate and then Mosaic as shown below.
It will ask you what square size you want, I use square size 25.
Then go to Filter -> Sharpen -> Sharpen. This step is not needed, but I think it makes it look better.
Set the layer to Soft Light.
Decrease the opacity or leave it at 100% opacity, depending on what you like.
Done!