If your one of those people who used to (or still do) find pinhole photography fun and have a digital slr (that should leave about two people on earth... me being one) you can make a very nice pinhole camera by putting a small hole (for most brands about 0.3mm diameter) in the center of your body cap then filing it back untill its as thin as you feel safe making it.

Not quite as cheap as a shoe box and a piece of film but if you have the equipment its a fun way to experiment... there are plenty of sites on the web with more info.

The equation for finding to optimum pinhole size (ie smallest circle of confusion while avoiding diffraction) is Diameter = 1.899 * sqrt( the distance from the sensor * 0.00055)

I noticed you can acctually buy a cap that has already been made. link. I have never seen much about it and was wondering if it produces an interesting effect or do people do it just cuz they can?

i think ill stick with the free way :-), as far as effects, depending on the size of the hole it can give a soft focus effect, or with a small hole it has the pontential to be much sharper then a glass lens, not to mention the unlimited depth of field, anything from touching the "lens" to infinity is in perfect sharp focus.