Anamorphic pictures are 2 dimensional pictures that, when viewed from a certain angle, appear to be 3 dimensional.
This concept is nothing new, The street artist Julien Beever has been doing it for years.
GreaseTattoo already has a great instructable on doing this technique as well as a ton of great examples of the type of things you can do with this. But there is no explanation on doing this technique in any other programs other than Corel Draw.
So after many hours mucking about with GIMP, then Inkscape, then GIMP again, then photoshop I bring you this instructable.
Step 1: Make A New Document and a Grid
Open up photoshop and make a new document to whatever size you want. I made mine
A4 so it would easily fit my printer paper size.
Now go to Image > Image Size and see how many pixels wide your document is and memorize it.
Now make another new document that is a division of your previously opened document.
An A4 document at 300ppi is 2480 pixels wide so I made my new document 248 pixels square.
Now go to Select > All (Ctrl+A) and then go to Edit > Stroke and select any color and make the width around 2 pixels.
Finally go to Select > Deselect (Ctrl+D) and then to Edit > Define Pattern and call it whatever you want and then close the grid square document (You don't have to save it).