a few options
I agree with the earlier post about staying away from full suspension bikes and disc brakes for less than $500.
A few bikes to consider would be: Trek 4300, Trek 4500, Gary Fisher Marlin, Specialized RockHopper, Specialized HardRock, Iron Horse Warrior, maybe a few Giants to choose from (I'm not too familiar with the Giant line). I would buy a bike from a big name company such as the ones I've listed above.
My biggest recommendation would be to go ride as many bikes as possible (even if it is a parking lot demo). You want to find a bike that fits you and that you are comfortable on. For example - Trek bikes have a more traditional feel whereas Fisher bikes (with Genesis geometry) have a more stretched out feel. Some people like this and others don't. Go to your local bike shop (LBS) and give a few bikes a spin. Don't be afraid to ask questions and try out different sizes of the same bike.
Hope this helps!