Sony, Pentax and Olympus make good cameras that are very capable to do photography. The issue with them is that they dont have a huge set of pro level lenses and bodies. If you want to do general shooting of your kids at the park or on a trip, all three are very capable. I believe its the Sony A850 that is a full frame, 24 mp camera that has image stabilization built IN to the body.
The issues with going with those 3 are accessibility of used gear. Most people shoot Canon or Nikon, making the used market pretty popular.
And in terms of pro, high end lenses and bodies, they try to compete but Nikon and Canon offer such great high end products.
So which to go for? Depends on your needs. I went with Canon. First reason is that I liked the look and feel of the Canon. Go in the store and hold and play with the menus for the manufacturers you are interested in. You will upgrade the body with 2-3 years, you'll keep the good lenses for 10+. So the body isn't the important thing, just make sure you are comfortable with it.
My other reason for going Canon was that a friend of mine shoots Canon and has some L pro lenses. So I have access to the 2k+ lenses when I need them, which I wouldnt get if i went with Nikon
If you want something cheap to start off with, look for a used Canon 40D. They are amazing cameras. They are the mid level range for Canon. I see them going for $600 used on Craigslist. If you want new, it all depends on the budget. The Rebel line is a good entry point, but if you pursue photography seriously, you will outgrow it quickly. The 7D is a great camera, but its 2k. Then you get into the full frame 5D MkII which is 3k and then the 1D line which is 5k.
Invest more in the lenses than the body, they make such a huge difference