Wow... Well, there are a lot of filters out there that are used for a whole variety of purposes, but to cover the high points:

• UV Filters are designed to reduce or eliminate UV rays coming through the lens. At this point, they're primarily used as lens protection, though the utility of that is debatable (I side on the not so useful side of that argument, but others disagree) versus the possible side effects (such as ghost lights). So, my rating is a 1 there, but that's personal choice and you have to decide if the 'protection' is relevant to you versus the potential side effects.

• Color correcting filters are designed to help create the correct white balance. For example, to color correct for tungsten light, you might use a 85B filter and that will help ensure that white is, in fact, white when you take the shot. My rating is, again, low, perhaps around a 2 here because white balance is easily corrected if you shoot RAW. This is a film holdover.

• A Polarizer is designed to cut certain types of glare. There are two types: linear and circular. They are basically the same, a filter that can be rotated to block light perpendicular to the filter axis which, in turn, can cut glare. For modern cameras, the circular polarizer is the one you want as the linear one may interfere with the autofocus function. Bear in mind that when using one, you will lose a couple of stops of light. My rating is a 5 if you ever want to shoot subjects around and through water or glass.

• Neutral Density filters are designed to reduce light, an action often seen as counter-productive to an entire practice of capturing light. However, the use of ND filters allows for significantly reducing shutter speed under very bright light without affecting the color of the light. Very handy if you want to slow down things like a waterfall to create that dreamy look as the water cascades through the rocks. ND filters come in a variety of strengths and it is a common practice to stack various types as needed to increase the strength. This is a 4 for me because I like water shots of all kinds.

• Graduated Neutral Density filters are similar to the above except that they reduce their density as they progress. As with standard ND, it's about light reduction, but intended to be more controlled so that, for example, you can reduce the brightness of the sky in order to even out with the brightness of the land. Again, there are various levels of strength, but unlike most of the above, these are best when they are not circular and directly affixed to the lens. You want a filter that is much larger than the front of the lens so that you can adjust the position of the gradation as you need for the shot. Cokin makes a good system for this purpose, though you are not limited to their filters if you get the system. Also a 4, though most useful in the landscape world which I don't do a lot of.

• Special Effects filters run the gamut, including options like a starlight filter that creates a 'star' like effect on light sources. These are all over the map and some can be fun, though you should bear in mind that many are gimicky. Maybe a 3 because of the fun factor, some of which you can achieve through other options such as creative bokeh kits. Still, with the right subject, a special effect filter can produce a cool result.

As for all of the above, in regards to price, you get what you pay for to a large degree and it is really all over the map. Better filters aren't cheap and, if you are getting them, it's better to pay the price knowing what you want from it. Having said that, options like the Cokin filter system can really help if you have a large lens collection. I went the Cokin route because I have 5 different filter sizes to deal with and I didn't want to be buying certain filters 5 times. Now, Cokin filters are very good, but not great, though the good news is that there are third party manufacturers that produce filters for the system and some of those really are great. I like the system, your mileage may vary.