Using Laptop for PC monitor

Help! I am trying to find and buy a product which can be loaded onto a laptop (and perhaps PC) so that a laptop can substitute for a PC monitor. Ideally this would plug into the PC’s video socket, but it could attach to the parallel, serial or USB.

I have neither room for a humongous CRT monitor nor money for a flat screen monitor, but I have several laptops and would like to be able to use them instead. I would really like to find a product which can let a laptop be a monitor for Linux as well as Windows.

Because of the frequency of terms like “monitor” and “laptop”, searching for this product has not been feasible.