I know my bike may not be the choice of many on this forum but I love my bike. I have a diamondback insight and it is under 500 dollars and it was a big upgrade to me from my Walmart mongoose mountain bike. I just got the bike last week I think and I've out in 70 miles on it and I haven't had a problem. Here is a picture of the bike
I've also have saw a fuji newest 4.0 at sport chalet for under 500 dollars. Good luck on your search for a bike.