Right now you can get a Nikon D3100 under $500, and there's no question that Nikon makes great DSLRs.
What you have to remember when buying a DSLR is that the expense of the body and kit lens is not going to be your only expense if you plan to get seriously involved in photography. If you want a DSLR because you're interested in the flexibility of multiple lenses and accessories, your choice of manufacturer is also going to impact your future expenses. Professional Nikon cameras and lenses are often more expensive than other brands, including Canon.
Right now, Sony trails Nikon and Canon in the DSLR market in terms of units sold, but you can get more features in a Sony camera for less than you'd pay for a Canon or Nikon. Are they good? Well, Popular Photography has chosen a Sony for its Camera of the Year in two of the last five years, and Nikon actually uses their sensors in their APS-C cameras. So, yeah, they're good. They just haven't been making DSLRs long enough to have as much name recognition in that segment of the market as Canon and Nikon.
I personally shoot Canon because that's what I've owned for over 30 years, and I absolutely love, love, love my Canon cameras (an EOS 7D and old T1i). But if I were just starting out again, it wouldn't be a slam dunk for Canon. There are so many amazing options on the market right now. I think I'd have a hard time deciding!
If budget were a main consideration, I would definitely lean toward Sony. Or even Pentax. You can get a completely weather-sealed Pentax DSLR for under $1000, and, yes, it gets good ratings.
I'm partial to Canon, of course, and think the EOS Rebel series is a great entry-level DSLR series. The T3i, T4i, and T5i are all great, but they're more expensive than Sony, Pentax and even the Nikon D3100.
But I think you'll get the most bang for your buck with Sony. (Sorry, Canon!!)
You can get a Sony A37K with a kit lens for under $500 on Amazon. It has more focal points than the Nikon D3100 (15 vs 11), can shoot faster than the Nikon (7fps vs 3fps) and has higher resolution than the Nikon (16.1 mp vs 14.2 mp).
Ultimately, however, your decision should be based not only on what you want right now, but what you plan on doing in the future. If you're interested in photography as a career or serious hobby, the choice you make now will also affect your choices - and expenses - in the future.