You put this ridiculous helmet over your head that has these 3D TV goggles, and when you move your head, it's like you're actually moving your head in the game.
Here's how to do something like that for under 100 bucks.
Camcorder with Viewfinder and video input
Gyro mouse (More on that later)
Normal wireless mouse (optional)
Step 1: About the Gyro Mouse
I got mine for 40 bucks at Circuit City. I've been looking around online, and that was a really good deal. You can use any kind of gyro mouse, but it needs to be able to do these things:
Pointer is based on tilting and rotating, not actually pointing at the screen. AKA no sensor bars or things like that.
It needs to be wireless.
If you don't have one try to find one here.
If you can't find one, this instructable will be impossible.
Step 2: The Camcorder
It has a viewfinder
It has video IN
It's not crazy heavy. You need to be able to hold it.
Step 3: Putting it together
Then, duct tape the mouse right side up on the camcorder. Make sure it doesn't fall off.
If you have trouble pressing the mouse buttons, duct tape another mouse in a better area on the camera.
Connect the camera to the computer with an S-Video cable or RCA cables, or whatever you have on your computer. (If you have nothing, put everything away, because you can't go any further.)
Step 4: Using it in a game
In the game (Which should be a first person shooter. I can't imagine why you'd want to use it for anything else) you need to mess with the settings to your liking. I like having to actually turn my head 180 degrees to look behind me, but if you don't, that's okay. If there's a setting for mouse sensitivity, change it. For more lifelike movement, make it less sensitive. For lazy movement, more.
If the pointer isn't right (Like, you're looking up in real life, but the game is looking foward) take the duct tape off the activation button on the mouse, center the camera, and put it back on.
Making it better. (Yes that's right, it can get even better!)
I know this setup is okay, but it's hard to control because you still need to use the keyboard. If you have a wireless keyboard, try that. I don't have one, so I can't.
Also, actually using video glasses would be awesome, but those are expensive.
My viewfinder is black and white. Color would be much better.
Maybe someday I'll upload a video of it's awesomeness.