Yep, also if you are shooting in black and white you can use a red and a green filter to darken and slow things down. I love going out on sunny days with my ND8, a polariser, a red and a green filter to really slow things down, use a tripod and you can get some really strange effects. BTW if you use two polarisers together then you can then control the amount of effect it has.

No because offset polarizing filters do not filter all wavelengths of light to the same degree. The most visible differences are in the shortest wavelengths, (blue towards violet), which tends to be effected much less than the shorter, (red), wavelengths of light. You can see evidence of this if you set the polarizer’s at a 90 degree angle to each other and look at something like a bright light. You will see that the blue’s come though much more strongly than other colors. Neutral density filters are well engineered to reduce all colors to the same degree and if true colors are important to you that is the filter you want to use when you want to extend you exposure times. Even if you are shooting Black and White there is a difference. Blue may not be a color in a Black and White picture, but it is a shade of grey. If it is shown as brighter than it actually should be, (which is what offset polarizing filters will do), than things that are that color will appear whiter than they should, relative to the other colors in the same picture. If you care about that, then you should use a Neutral density filter. Using a colored filter would even worsen the problem and be much worse than using two polarizing filters. A red filter will make things that are green to look black and a green filter make things that are red look black. Unless you want to distort the grayscale appearance of different colors in your black and white picture, you should not use a colored filter as the previous author suggested.

All answers above are good (esp. Anamouse's caveats), just wanted to throw in the obvious: you have to combine a linear polarizer as first element and a circular as second (in almost all cases a circular is better with respect to the camera, esp. SLR). If you screw two circulars together...well, it won't work, right?