Raycast from camera on mousedown.

I need a way to initiate a raycast from the camera when the player uses the mouse. Kind of like a laser, only once per click. I was trying to get together a method using rigidbodies, but the collision was faulty, and it was too slow.

Take a look here: http://unity3d.com/support/documentation/ScriptReference/Camera.ScreenPointToRay.html

Code there sounds like what you need