Ah, they missed a few really good filters. There is the darkness filter, which filters out the dark and lets the light get through. Depending on how much dark absorption you get, you can get anywhere from 2 to 6 stops of additional light. But you have to be careful to clean out the darkness filters from time to time, otherwise the darkness saturates the filter and then you get all kind of annoying dark leaks. It takes forever to clean out the dark leak in your camera bag.
Then, there is the noise filter. It allows you to filter out the noise in the light, and you get clean shots even when you bump up the ISO. However, if you're not careful, that noise filter also attracts static, so you have to regularly wipe it with anti-static fluid.
Now if you're having issues with focusing, you need the split ring focus filter, which works like trifocals for your lens - simultaneously focusing on near, medium and far points. It's a neat effect, and totally confuses the other photographers who can't figure out how you get multiple zones of DOF. They think it is photoshop, but you know otherwise.
Oh, and don't forget the helium filter, which allows you to lighten the front of some of those heavy lenses. But make sure to buy the good helium and not the hydrogen filter, which can explode if there are sparks around. Check that the ring reads "He" and not just "H".
What? :mrgreen: