mosaic effect
This also is a very easy one. First of all you need a picture. I chose this one:
Now duplicate the layer. Now go to: Filter -> Pixelate -> Mosaic. The cell size depends on the size of the picture. I chose 10, but on bigger pics you can also take bigger cells. Your picture should be looking like this now:
Now you should set the layer on "Screen" (which I chose) or "Soft Light" (or whatever you like best). If you don't like the way the cells are seen, you can change the layer's opacity (I set it on the second picture on 75%). Now this is your result:
or 