The Only Camera Filters You Need
In the old days photographers carried around a bunch of colored filters. You don't have to do that anymore. Photoshop® has them all built in and you just pick your filter later on the computer. Most digital cameras have some digital filters built in. Why use filters? The main reason is for black and white photographs. Using a yellow filter with black and white can improve the contrast, making a blue sky darker. A green filter can improve the greens of plants, but it doesn't affect skin tones. Filters can also be used for special effects in your color photographs.
Camera filters - Photograph black and white normal, with yellow, green filters
Two filters I still use are a UV (ultra light) filter to protect my lens, and a polarizing filter. A polarizing filter helps eliminate reflective lighting of windows, and water surfaces. Light vibrates after reflecting off non-metallic surfaces like glass and water. The polarizing filter filters out a lot of these reflections. Polarizing filters can also make the sky look dark blue and cut the haze by filtering out the reflections from water vapor in the air. Make sure to get a circular polarizing filter so the filter works with your auto focusing. Also, for best results, take the photograph 90 degrees from the sun. Photoshop© can't do polarizing. Be careful of polarizing filters with a lot of the sky showing, when you use wide angle lenses. Polarizing filters can cause a dark blue streak to appear. Rotate the polarizing filter until it gives the effect you want. Be careful to rotate it in the direction you would normally tighten the filter, since you might accidentally unscrew the filter as I have almost done before! The sky is going to turn out a lot more blue then your eye sees it. You might not want to have a super blue sky! I like the way it looks, but this can be overdone. Polarizing filters can dramatically change outdoor photography by making colors more vibrant. Your ISO setting might need to be boosted, because a polarizing filter cuts off about 1-2 f stops of light.
Why get a UV filter? The UV filter can improve the saturation of colors and cut the haze a little. The main function of a UV filter is to protect the front of your lens. I have more then once dropped a lens which just shattered a filter and just replaced it for $20. This is much better then having to trash the whole lens!
Close up filters are very useful also. See the site macro-photography.org for more information on this.