Photographers often think incorrectly that PhotoShop will save their hides. If you can't take a good photo in the first place because you don't know how to use your gear, you are a techie, not a photographer.

Polarizers are VERY important. They can be up to $80, but usually if you buy a Cokin or Tiffen filter set, you can skate away for far less. A polarizer not only cuts glare of cars, water and glass, but darkens the skies to make coulds whiter. It also helps out flesh tones quite well. The only downside it that they can cut the amount of light in to the camera so in lower light they are not useful and never in doors.

Warming filters help make skin look good, and even help saturate the colors of everything around you. I rarely take mine off, I usually tell people to always keep it on the lens.

Graduated (aka Gradient) filters have a fantastic effect. You can take pictures of people infront of a sunset or extremely bright sky. You slowly slide the filter in to produce that great effect. With a tobacco colored one, you can give people a nice tan and make a fantastic sunset colored sky any time of day.

UV filters don't do squat. They are important to have to prevent damaging the objective lens, which is the first piece of glass (and now more often plastic) on a lens. Since many lenses are now plastic, this is useless. If you bump it enough to break the filter, you have probably broken the gears in the lens and now since plastic is softer than the glass of the filter, your lens is gouged. Buy a filter you can use, not some snakeoil.