Polarizer filters can be used to enhance contrast and saturation in landscape photography and reduce reflections or glare on shiny surfaces such as water. The effect is maximized by aligning the polarizer on the camera so that the polarizing direction is perpendicular to that of the light you want to block.
On a bright day it works best if you are in a 90 degrees angle to the sun, but there will always be some light reflected by the surrounding or diffused by clouds which still reduce the effect. (See the differnce between the right and left sides of the image below)
This article shows how a polarizing filter for a flash allows to make use of the effect with artificial light. Some subjects will look gorgeous while others may be rendered in an odd way but it's certainly fun playing with the effect and you can still vary the level by turning your filter on the lens. In a nutshell, we are going to polerize the light coming from the strobe to better control its specular higlights.
Every lightsource could be modified to emmit polarized light. But some devices (such as displays and some lasers) already emmit polarized light. See the images below for examle images illuminated with an iPad.