I saw a picture of a hidden wall safe on http://www.lifehacker.com and found it was a product being sold for about $8.00 from ThinkGeek.com.
Rather than order one, it seemed feasible to make one with just a couple of materials from the hardware store. Not to mention, much cheaper.
There are so many variations that could be made to this idea. I demonstrated 4 that I came up with, and perhaps the best one was the fact that you could just use the box behind your cable. If you use one of those, it's completely free, and you don't have to cut any new holes in your walls.
WARNING: Extreme caution should be used when cutting into sheetrock. There may be electrical wires, or other sensitive materials behind the wall that can't be seen, and could be damaged when cutting. Use of video content is at own risk.