Yet again, Canon must have seen me coming: I've spent £160 on a small pane of glass. The lens I tend to use on my camera is the 70-200 2.8 IS, which is a great all-round camera for the kind of shoots I do. The downside to such a fast lens is that it requires huge filters, hence £160 for a circular polariser. Despite being pricy, these are cool, but first I'll explain what these things actually do if you're not into photography (and yet, for some reason, still care what a polarising filter is...)
Light bounces everywhere around us, creating lovely soft glows even in places that don't get light direct from the sun. This is usually pretty cool, as long as you keep a lens hood around to counter the contrast loss. However, in some circumstances this light can become polarised: it vibrates along only one plane, as opposed to randomly.
All this polarised light causes extreme glare from your subjects, particularly when you're trying to photography glass or water. The interesting thing is that polarised light is also very common in the sky, where teeny little particles in the air cause polarisation and make the sky look hazy as a result.
Polarising filters block out - ta da - polarised light, which means by using them you can get some great pictures of the sky because the outlines of the clouds become much more clearly defined. Here are some pics to make it clearer, taken as a snapshot in Bath at lunchtime. Both were taken within a few seconds of each other.
The effect depends largely on the weather in which you're shooting - if the sky is bluer, the effect is stronger. This sky was quite pale, but you can still see the polarised picture darkens the sky considerably, whilst also levelling out the reflected light in the building.
Particularly of interest is the main window: in the unpolarised picture it's lighter, whereas it's darker in the polarised picture. Again, this is the reflection being filtered out - if there were a person in there, they would be a lot clear with the filter on.