CanoNikon

CanoNikon is the term most often used to describe the rivalry between fans of Canon and Nikon. Both companies are the worldwide leaders when it comes to modern 35mm cameras and for good reason; the quality of their bodies and lenses rank among the best anywhere. Professionals use them for portraiture, sports, photojournalism, etc. Visit any country and one of either will have a large presence. It's the same in my locality where the two are neck-and-neck in terms of sales.
Other companies like Pentax and Olympus, although very established camera brands, have only recently become more aggressive in terms of DSLR innovations. This has allowed newcomers like Panasonic and Samsung to come in and take a slice of the pie.

Recently, I've seen some photographers sport Sony bodies and lenses, although they are the exception as Sony's prices are too prohibitive to take a bigger share of the market; the quality of their cameras is quite good, but at their current prices, you would expect something quite spectacular. Nevertheless, Sony's marketing team here has done a good job to attract a steady base of customers.
With these newer brands coming in, it's difficult to make a more objective decision, especially if you have no idea what to look for in a camera. Personally, I went back and forth between Canon and Nikon and finally (and happily) settled with the Canon 450D years ago for a few reasons... 
1) Canon cameras feel comfortable in my hands.

Our family had a beat-up Nikon point-and-shoot film camera several years ago held together by tape and sentimentality. We continue to use this camera up until digital cameras became popular.
When the film camera finally stopped working, we sprang for the Canon SX100, a higher-end point-and-shoot digicam. In my opinion, the transition from film to digital was illuminating; this was the camera that awakened the photographer in me. 

For the price, it was an amazing camera: 10x zoom, 8 megapixel resolution, auto and scene modes, and best of all, it had full manual controls. As a beginner photographer, I thought the auto mode on sunny days was excellent and its low light ability was great for its time.

As I became more and more adept, the manual controls made for a much more fun experience when learning about aperture, shutter speed, ISO and other photography basics.

When I decided to upgrade from the SX100 to the 450D DSLR, the choice was easy; the Canon cameras' controls were similarly laid out, comfortable and very intuitive.

2) Most of my friends have Canon gear.

This can actually be a big influence when you're asking for advice offline or if you've purchased a different branded camera. It can be very frustrating to be the only one in your group dealing with technical problems specific to your model.
For example, a friend of mine recently bought a Sony DSLR (it was an older model and was on sale). It was an excellent camera and it felt much more comfortable in my hand than my own Canon 450D. However, since he was used to handling my Canon camera months before, he had a difficult time adjusting to his own unit. I helped him out whenever I could, but there was very little I could do with my limited knowledge of Sony's cameras and lenses.

In contrast, I have a much more fun time when I'm with friends who have Canon bodies. It's something you don't notice when you're browsing in the store or over the Internet, but the fact that you're in the same-feathered group can matter a great deal in the long run.

3) Canon has a service center in our town. 

Looking around for my first camera was easy because of the number of photography shops in the locality. In particular, having a Canon showroom and service center proved to be the icing on the cake. The staff members there were helpful and knowledgeable, and assured me that if anything ever happened to my camera, I could take it back and have it repaired for free (with the warranty).

Although nothing serious has required me to take my unit back, it's nice to know that the service center is there. Like they say, it's better to have it and not need it than to need it and not have it. From time to time, the center also offers free cleaning services and routine inspections, a big plus for their marketing. 

In the past two years, Sony and Panasonic both opened service centers in the area that also cater to consumer and professional DSLRs, although their presence is mostly due to their local sales of TV screens, entertainment centers and other electronics.

If you own a Nikon camera and it suddenly stops working, I have no idea where you can have it fixed in my town.

On a related note, I find the current generation of Canon DSLR bodies and lenses much more aesthetically crafted compared to their Nikon counterparts, but I think that's just a personal bias that's grown throughout the years I've had my own Canon 450D.

If you're shopping around for a new camera, both Canon and Nikon are excellent brands, and have fantastic track records of quality products. It will take a few more years for the other brands to consistently prove themselves with their own line of cameras, but that shouldn't stop you from choosing any of these.

Google about the latest camera reviews, ask your friends and try them out for yourself. That's the only real way you'll find the camera meant for you.