Moubtain Bike Under $ 500 !!!!!!

Dude, if you've only got 500 to spend, go used. The buy/sell here is kinda tough, because you can't really filter anything in the forum, and most of the bikes are pretty highly priced (not that it's a bad thing, per se). I'd check out Pinkbike for that aspect of it. I've bought and sold items/bikes there and never had a problem; both shipping items (use paypal) and in person transactions Just a quick search turned up a few decent little bikes:

Trek 6000 - $500

Norco Bigfoot - $500

'04 Specialized Enduro - $500

New though? I've always been a fan of GT, you could pick up an aggressor for under 500 (460), but if you could spring for their top version (550, which upgrades to hydro brakes), that'd probably be best. Honestly, most of the major companies are going to have pretty similar bikes at that price point... Usually a low end Suntour/RST/"house brand" fork, Shimano Altus/Tourney level components, Weinmann level rims and loose ball bearings for hubs, tektro cable disc brakes, etc.

GT Aggressor Comp - $460

Giant ATX 27.5 - $460

Fuji Nevada - $500

Specialized Hardrock Disc - $500

Trek Marlin - $500

At that point, I'd go with whichever shop you prefer, and with what brand they carry, to be honest.

I've been riding for a long time, and I've only ever bought 1 new bike... that includes the times I worked at shops, even (for 5 years). Total I've probably owned at least 12 of them. Used is just the way to go, IMO.

Trek 4500 or 4300 Alpha

Specialized Hardrock or RockHopper

Diamondback Sorrento or Response

Giant, Marin,GT,Scott....all of these can be picked up used for well under $500 on Craiglist with money left over for personal upgrades.