Alright so Gunjan nearly got it in one yesterday! My second must-have accessory is a polarizing filter. Think of it as a pair of sunglasses for your camera.
The most common use for a polarizing filter is as you see here. It really darkens blue skies and brings out the white in the clouds. It also makes colours pop.
This was taken at a place called Arltunga, which was the first pioneer settlement of Central Australia. As you can see the day was screaming out for the polarizing filter.
The trick with these filters is to control how strong you make the effect. The front of the polarizer spins and as you rotate it you can actually see the polarization effect getting stronger and weaker in turns. If you put it at full strength in these kind of conditions you can turn your sky to an ugly black. The trick is to dial it to full strength and then back it off a little bit to retain a natural feel.
But, you say, that picture yesterday didn't have any blue sky in it at all. No it didn't, but it did have lots of reflections. The other thing the polarizer is used for is to cut down on reflections. Think streams where you can see the fish in the water with your sunglasses on but take them off and the reflections of the water don't let you see below.
Now in yesterday's pic it wasn't the reflection in the water I was trying to cut out. It was the reflection from the leaves. In warm, tropical areas the leaves of the plants tend to be large and flat. The large surface area tends to work like a giant mirror sometimes and the reflections are really strong. It's the main cause for less than vibrant colours in rainforest pictures. Using the polarizer helps get rid of the reflections and bring the brilliant greens back to your pictures.
Now there are a couple of different kinds of polarizers - circular and linear. Only the circular ones will work with the auto-focus on your camera so that's the one to get. You can also get them in different thicknesses. There is the normal one and a special thin one which is designed for use with wide-angle lenses. The regular thickness polarizer can tend to vignette at the corners of the frame on really wide-angle lenses. This effect is where you can see the edge of the filter in the corners of your picture. The thin polarizer is designed to negate this problem, but it comes at a price as they are quite a bit more expensive.
So there's my second must-have photographic accessory. I don't tend to carry many filters with me these days as the White Balance function has taken away the need for a lot of them, but I'm never without my polarizers - one for every lens I own.