OK, the list prices for my two votes definitely don't fall within the $500 threshold, but thumbs up to the Raleigh M80 and used Specialized Stumpjumpers.

a previous season model of the Raleigh M80 can be found for that price. As a newbie to MTB, I found it to be a great bike for getting into the sport and doubling as commuting bike. It didn't seem so expensive-looking that it would be singled out by thieves. Well, that was before my M80 disappeared along with my Kryptonite u-lock one day.

Anyway, so to replace the M80 that had been stolen with either several thousand pounds of force or a bic pen, I found a barely-used 2000 Specialized Stumpjumper M2 Comp on eBay for between $500 and $600. Such a big difference in the ride compared to anything that lists for $500!

So my point is, if you're looking in the $500 rather than $300 range, consider buying used. There are whole a lot of people who have $ to burn, put down $1K for a bike they take to the trails once or twice before deciding biking is not for them, and then realize later they have some pristine bike rotting in their garage. Their loss can be your gain.