For me, one of the coolest things to do with Photoshop is crazy, bizarre photomanipulation. Joining a monkey's head to a human body can be tricky, but that's no problem to Human Descent, a great "digital geneticist".
Some of these are so real that one can almost say it's the true thing... that's only because this dude make one hell of a job making these. For more of his manips, just make a visit to his Flickr photostream. You'll have fun for sure! Cheers! ;)