You might have seen this effect used on graphics across the web, it is a very popular technique. It is also very easy to create, and once it is done, you can use it over and over again without having to re-create it.
1. Create a new document (File--> New) with the contents set to transparent and with the width and height of 4 pixels.
2. Use the Zoom tool (Z) to zoom in to 1600%. Get the pencil tool (B) and with a 1px brush and whatever colour u want (it looks best black though) colour in all the pixels down the left hand side of the image and all the pixels down the bottom of the image. Here is what you should have so far (the image below is shown at the zoom of 1600%):
3. Next, press ctrl+A to select the whole picture and go to edit-->define pattern. Give your grid a name, like grid for example.
4. Open up a new image (or an old one that you wanted to do this effect on) and make a new layer then go edit-->fill. In the box that pops up, select Pattern In the Use dropdown menu, and underneath it select your newly created pattern.
5. Lastly, lower the opacity of the layer that the grid is on to 30%-50% and it should look great.
I used this same technique on an old image that I had and came up with this:
