You pretty much won't be able to find a new, current-gen DSLR by any of the major manufacturers for under $500. However, since Nikon have already released three new models this year, you will likely able to find previous generation models for a pretty reasonable price.
While not truly a DSLR it certainly shoots like one, the Lumix G3 is actually an AMAZING camera for the price. Might be something worth looking into. The only real difference is that is has no mirror so the viewfinder is electronic instead of optical. [link]
plus $400 puts you well under budget so you can start saving for a high end lens for when you outgrow the kit lens that it comes with.
You don't necessarily have to get a new model. You could easily pick up a DSLR off eBay for under that price - the Canon Rebel XS comes to mind, and I'm sure there will also be a Nikon equivalent available that way.
any new model's by nikon or canon are nice cameras. But I'm not sure there are any for 500, Canon might have one for like 550 and I think Nikon for like 600 at cheapest for new models. not sure on that though. I'd recommend nikon only because I use nikon and I know for a fact they are nice and all of the models are up-to-date. I'm not sure about canon but I'm assuming the same would apply.