[font color=\'#000000\']The quality and quantity of add on lenses from Nikon was a major factor in my decision to go with the Nikon 990 as my first digicam.
I bought Nikon's 2x extender and have been VERY happy with it. In actual use, I've found no difference in lens sharpness, color,or contrast with it on the camera. The front element is huge so you don't lose any lens speed when using it. You will lose low end zoom, however. To avoid vignette you are limited to about 120-225mm focal lengths.
I also added an Eagle Eye 5x lens to my kit. You do lose some lens speed with this one, and you have a rather restricted zoom range (almost no zoom, actually), but it is reasonably sharp across the frame and when 550mm is all that will get the shot...
In addition to the 2x, Nikon has a 3x, a wide angle (about 0.7x I believe), and a fisheye. They've recently added a LED based ring lite for macro work. I don't think anyone can touch them for completeness of kit in the digicam market and the quailty (and price) is what you would expect with the Nikon name.
Nikon has added a number of digicams since my 990. Some work with these accessories as is, some require adapter rings of one sort or another, and some probabley don't work well. So check the camera reviews if this is driving your decision.
Happy shooting,
Charlie[/font]