One place: Specialized. I'm very biased here, I admit, but I purchased a 2007 Hardrock Sport almost exactly two years ago for $350. I've done some work to it, sure, but mostly due to either lack of proper maintenance (chain, cassette, BB, crankset) or 3,000 miles under my 300 lb+ butt (cracked rear rim). Mine's undergoing reconstruction, soon to be reborn with a handbuilt Salsa Gordo rear wheel with a LX hub, Thomson seatpost and LX rear deraileur.
It's my second favorite bike ever, and if/when I purchase another MTB it's either going to be a Specialized 29er or a Surly karate monkey.
Giant makes good entry level MTB's, and I hear those who are Trek/Gary Fisher fans as well. Try to find a '08 model on closeout, most '09s are going up in price from any manufacturer.