I've been riding a Giant Iguana disc for years, the bike is pretty light and strong. Its a hardtail unlike VCrossfan's full suspension and just has the shocks up front. You have to decide what you want: full suspension or hardtail. Hardtails are usually cheaper and lighter, full suspensions usually have a smoother ride but can be pricy (and the cheapos are usually really heavy)- pretty much a recap on what Mark said.

I would recommend full suspension if you want to spend at least $300-400 for a used one and a hardtail (with front shocks) if you want a decent bike for cheap. I once had a $99 full suspension bike from walmart and after a few months the crank arm broke off and the rear suspension locked up.

If you find some bikes on craigslist, you should post them on this thread and we can let you know if they're any good looking.