What are some good filters for a DSLR camera?

I'm considering buying a filter for my Nikon D90. What are some good ones that you would recommend?

I'm looking for ones that improve the quality of the picture. I'm pretty new to anything to do with filters so any opinions or suggestions would be great!

-definitely get a UV filter to protect your lens
-Get a polarizing filter for saturated color and less weird reflections on water
-Neutral Density filters are like sunglasses for your camera - they help you use slower shutter speeds

As already mentioned, a UV filter will be a help, but to improve the quality of your pics, get to know your camera and use it properly, read the manual as that will help a great deal.
Filters DON't automatically make a bad pic good.
You have to know what you are doing first before using filters to add effects etc.