It has been a while since I've programmed in Cocoa. But recently I cobbled together my own image and video viewer. The goal was to allow the user to drag-and-drop images and videos (like from a website) into one big, convenient canvas. In this canvas, the images and videos could be easily arranged (by dragging) and resized (using the scroll wheel). Make it smooth and responsive, and also throw in Apple Speech Recognition to control video playback, and I'd be happy. I'm more or less happy with the result. The hardest part was freely arranging and resizing the videos (and maintaining their z-order) in the canvas.

The name of the app is Coriander Collage. (As a warning, the website contains adult content.)