Most filters (in front of lenses) that people try to sell you don’t make sense in digital photography. In particular UV filters, sky light filters or protective filters — the first two are irrelevant, and you usually protect your lens much better by using a lens hood. All that these “useless” filters do is degrade image quality.
The only two filters that make sense, I guess, are polarisers and neutral density (or just “grey-“) filters. I might talk about the latter at some other time, today’s topic is the polariser. If you don’t have one, get one. Unfortunately, the better ones (that’s the ones you want to buy, since they have decent coatings on them, are more colour neutral and usually live longer) can be quite pricey, especially when you have large lens diameters.
Why? Well, the main effect of these filters is one that you just can’t reproduce properly in post-processing: They reduce reflections. There are mainly two situations where this is important 1) in the sky and 2) on colourful objects (well, actually 3) on translucent surfaces). In the sky the filter gives you much deeper blues as it removes some of the sunlight reflected off the haze in the air (which leaves the deep blue rather “milky”). On objects, such as grass, leaves, buildings, etc. it reduces the reflection of the ambient light to again just give you the actual significantly more saturated colour of the object. Third, it allows you to shoot through windows or water surfaces to reveal what’s behind (again, by cancelling out reflections of the surrounding).
Sounds great, doens’t it? Well, there’s a catch or two. On the one hand, you lose about 1 to 3 stops of brightness. On the other, in order to be able to cancel reflections, you need to get the angles right (this has to do with the physics behind all this). Since the post is already long enough, I won’t go into explaining how exactly to use them — if you’re interested you can have a look at this article for instance, or this page.
As for the photo above — you guessed right, it’s been shot using a polarising filter. No post-processing really, just the filter’s magic :-)