In the early days of the polarising filter there was just one type – the linear polarizer.  The circular type arrived when the cameras with advanced metering (and later focusing) systems became popular. The Canon T90 and Olympus OM4 both had new advanced TTL metering that would be affected by the way a linear polarising filter worked. So a circular polarising filter had to be used instead.

Later, when autofocus came along, some systems got confused by the affect of the polariser. So, as a rule of thumb, use a linear on old manual and semi automatic cameras and circular on newer modern multi menu models.

If you do have a linear filter and a modern sophisticated digital SLR it will still work, but you may get inconsistent exposures. This is easily rectified by looking on the LCD preview and reshooting with necessary compensation applied.

If you have a computer with an LCD monitor and a camera with a polarizing filter on the lens you can create some really colourful photos like this:

All you do is place a piece of plastic (the above is a cd case with ice on the surface) in front of an LCD screen and photography it using a camera that has a polariser attached to the lens (or held in front of the lens)
By rotating the filter you can increase the strength (saturation) of colours.

Try with cd or cassette cases, plastic glass, plastic cutlery, geometry items, filter cases and other similar hard plastic items.
The patterns revealed show stress in the plastic.