What is a good mountain bike for someone who rides recreationally, and would use it for transportation <$600?

I would use it for getting to work and back. Hard tail. I have been looking but I don't know which brands are the quality brands.

Two quality brands for commuters are Trek and Electra. Make sure you go to a real bicycle shop and not a store like REI or Hudson Trail. The people who work at bike shops generally love biking and know their products really well. They will be able to recommend something for you, measure you for the right bike size, and often the bike maintenance is free after you buy from them. Plus they'll install all the after products you need like a rack for your stuff, a good headlight, etc. You may want saddle bags too if you're going to bring a change of clothes to work.

If you're going to be riding on roads at all (in addition to the hard trail), I'd want a bike where my feet can be flat on the ground at stoplights. The Electra Townie line is designed for this and I think some of the Trek bikes are too (but I'm not positive but a bike salesman would know). So call the bike shops around your area and go to one that sells both Trek and Electra models so you can see both. There may be other brands too and I don't mean to exclude them, but a good bike shop will go over all of them with you. Good luck.

Check out Gary Fisher bikes. He's kind of a mountain bike guru.