SomethingAwful regularly has very amusing competitions to make use of Photoshoppery to come up with some amusing pictures based on a theme. Their humour does tend slightly towards the puerile, and the quality of the images is less than what you would see at Worth1000, but sometimes there is some really great stuff.
Take for example this contest: Lights! Camera! Act-WHAT THE FUCK?!
You can see a small version of the Seven Samurai Band image right there, but do check out the contest to see the fullsize image, and make sure you also see Falling Down Hello Kitty, Gandalf The Crossing Guard, and The Emporer’s Throne