You are probably here because you are trying to decide whether to buy a Nikon or a Canon digital SLR and did a Google search along those lines. Well, you have come to the right place. This is a question that gets discussed a lot and well thought out conclusions have been reached. There is only one sensible way to make the decision. Dig a quarter out of your pocket (or bum one from somebody) and give it a flip. If it comes up heads go with Nikon, otherwise go with Canon.
Lots of excellent photographers use Nikon gear (Joe McNalley, John Shaw, maybe even Scott Kelby). Plenty of other great photographers (myself included) use Canon gear.
My choice was largely influenced by the fact that people I knew at the time were using Canon gear.
Makers other than Canon and Nikon are pretty much minor league (like AAA baseball).It is a good thing that we have both Canon and Nikon because they keep each other on their toes (competition is good!). People often cite Canon's strong point as being their selection of lenses. It is certainly true that once you invest in one system (which boils down to spending money on an arsenal of lenses), it is very hard to justify a switch.
Every now and then I hear of a shooter that is selling all his Canon gear and switching to Nikon, or vice versa, for some reason. This is senseless. The thing to do if you have one or the other is to get out there and take pictures.
Tom's Digital Photography Info / tom@mmto.org