Photographic filters

Photographic filters have been in use for a long time. They were required for special effects or to enhance the contrast of some colors. They were also used for white balance.
For example, a film set for daylight color temperature, would have produced yellowish photos when taken inside, with incandescent light bulbs lighting the scene. To compensate, a blue filter had to be used, to get the right colors.
The major advantage of digital cameras is the fact that you can adjust the white balance of a scene electronically, without the use of a color filter. As far as the image effects that could be obtained with some filters (like blurring the image, changing color hue for specific areas of a photograph, etc.) there are better ways of getting them onto a photo.
Very cheap image software can produce more effects on a photograph than all the filters in the world combined. With a little bit of imagination, using software to get the effects you want is the best way to go.
The only filter a digital camera actually needs is a polarizer.
Polarizers come in two varieties:Linear polarizers Circular polarizers
Each has the same effect but circular polarizers are more expensive. Digital cameras can use both (linear and circular) polarizers.
Polarizers produce deeper colored blue skies, minimize light reflections from glass and water and reduces glare from non-metallic surfaces. They also provide good color saturation. Can be used in extremely bright light situations to reduce the amount of light entering the camera; this enables more selective depth of field control.