Just a fun game I remembered from the old times on the PoTW forums. Photoshop tennis. It's very simple:
-Someone posts a stock
-Someone has an idea for a cool effect (not to much though). He makes a post and says "working on it". When he's done he posts it and then someone else can add an effect. An so one and so one.
Some examples here. I especially recommend checking out the Danielle Russell vs LJ Jun one that's just brilliant.
What about this stock for starters.