Builders have been using the French Cleat for a long while. It's an easy, strong and versatile mounting system that, while most commonly used to hang cabinets, can be used to mount nearly anything.
That "nearly anything" includes flat-screen TVs, if you ask one helpful commenter at the This Old House online advice forums:
"There is an age-old, inexpensive, method for mounting cabinets and other heavy items that also works well for wall mounting flat screens. It is called a "French Cleat." The top block is screwed to the hanging object. The other half is attached to the wall. When you place the top half of the cleat over the bottom half, the item is locked firmly in position."
That's a diagram of a French Cleat at left. Assuming that both beveled blocks of wood are securely mounted to both the wall and the television, this thing can handle the weight just as well as a traditional wall mount for a tiny fraction of the cost.
If you need to tilt or swivel your TV, you'll need to buy a manufactured mount. But if you're just looking for a cheap way to hang your flat screen in the living room, this idea works—and it even keeps the TV closer to the wall for better cable hiding!
To get started with this project, check out directions and pictures of his French Cleat TV-mount setup from the commenter at This Old House.