The Cookie Cutter tool
The Cookie Cutter tool is a cropping tool that works just like a baker's cookie cutter. Drag a custom shape over your image and the cutter removes any pixels that are not inside the cutter shape - there are hundreds of shapes to choose from.
Some of the cutting shapes are great fun—I particularly like the rough-edged photo Crop Shapes because they produce a completely different photo edge from the normal standard rectilinear photo edge.
Above: Here's a classic Crop Shape effect. You'll find the Cookie Cutter on the Tool Bar, sharing space with the regular Crop Tool and the Perspective Crop tool (a feature that combines cropping with perspective correction).
Choose your cutter shape, then click and drag somewhere in the image. This takes a bit of practice, both in the dragging action and image choice but, as long as you have not pressed the OK (green) button at the edge of the cropping marquee to action...