This is a great effect that is surprisingly easy! Great for corner effects in your images.
1. Open up an new image (file-->new) with the width 300px and the height 300px and press ok.
2. Press d then x to reset the colours.
3. Press ctrl+backspace to fill the selection with black.
4. Select the elliptical marquee tool (shown below) and give it a fixed size width of 100px and a height of 100px (also shown below).
5. Make a selection roughly in the middle of your page and press d on your keyboard to reset the colours, then press ctrl+backspace to fill your selection with white. Then press ctrl+d to deselect.
6. Next, go to filter-->stylize-->extrude and enter the settings:
Type: Pyramids
Size: 5 Pixels
Depth: 100 and select the random option
Don't click the mask incomplete blocks box.
You could leave it there and press ctrl+u to change the colours if you wanted, or you could go on to enhance the effect by pressing ctrl+f 3 times to get longer spikes. Then you could press ctrl+e to adjust the hue and saturation (making sure the colourize box is checked).
By pressing ctrl+f the other 3 times and adjusting the hue and saturation, i came up with this:
It can be easily applied to images to result in something like this:
