Some popular review sites include
Steve's Digicams and
DPReview.
The major manufacturers of DSLRs are Canon, Nikon, Sony, Pentax, and Olympus. The first four are pretty comparable. Olympus uses a smaller sensor which makes for a lighter, more compact system but at a trade-off in image quality.
Each of the systems has its strengths and weaknesses. They are incompatible with each other; so you can only buy lenses designed to work for the system you buy into.
All of them are amazingly good compared to anything available at any price just a few years ago.
When looking at systems, consider the total system cost, not just the cost of the camera. In addition to the camera, you need to buy lenses, memory cards, and possibly other accessories like a flash, tripod, filters, spare batteries, etc. It's very easy to spend more money on lenses and accessories than you did on the camera itself.
Each manufacturer has several quality levels of cameras from beginner to professional. The costs rise exponentially as you move up the quality ladder, but the benefits increase quite slowly. There is a much, much larger difference between the best Canon P&S and the entry level Canon DSLR (Rebel series) than there is between the entry level Canon and professional Canon DSLRs.
I would advise sticking with the entry level camera for the manufacturer you pick unless there are features up the ladder that you are convinced will pay for themselves in a few years time. The reason is that camera bodies are improving at a very rapid rate, so you are likely to replace the body in 3-5 years. On the other hand, lenses, flashes, tripods, and other accessories evolve much more slowly. Because of that, if I had enough money to buy more than just the entry level DSLR and kit lens, I'd put the extra money into more/better lenses or accessories rather than a better camera body.
Whichever camera manufacturer you choose, you'll find capable shooters here that can help you with any manufacturer specific questions you might have.
I would also recommend that you try to play with the different cameras rather than just buy them based on reviews. Everyone has different hands and some cameras fit better than others. Everyone has a different sense for how a camera should work and each manufacturer has different ways of doing the same thing. A typical big box electronics retailer will have some Canon, Nikon, and Sony gear. I don't see the Pentax or Olympus as often in the stores where I live, but I'm sure someone here can point you to some retailers for them.
One other consideration I find important (although not everyone does), is what your friends shoot with. If you have serious photographer friends, using the same type of gear that they do opens you to more opportunities for help and for borrowing/buying/trading gear.
I think the cameras you should look at first are the Canon Rebel XTi, the Nikon D40x and D80, the Sony A100, the Pentax K100D, E-410. You should also check the "big brother" of any of those cameras that interest you (like the Canon 40D, the Nikon D300, the Sony A700, the Pentax K10D, and the Olympus E-510). That will give you a sense of what the entry level cameras cannot do and will help you with assess whether it is worth it to spend a lot more money to get a few extra features.
Don't be too quick to judge cameras by the sample shots that you might see here. We have some photographers here that I swear could shoot drool worth shots with a cardboard pinhole camera and others that make really great cameras look pretty mediocre.
One other thing to remember is that even cheap cameras can get great shots when the conditions are right. The same is true for mediocre lenses. More expensive gear shows its advantages when conditions aren't good (low light, lots of movement, etc). I just got back from a trip during which I used three cameras ranging in cost from $250 to $4,500. Some of the shots from the cheapest camera look as good as the shots from the most expensive. On the other hand, some of the shots from the most expensive camera couldn't have been taken with the cheapest camera no matter how talented the photographer was.
