Ardour is probably the most famous open source (hence : free) DAW on the market but I really don't like it. Do you really need it to be free ? When do you need it ? We've got a DAW under development. We plan to release a beta version in a couple of months and it's not restricted (you can save, export and do whatever you'd do with a full version), but because it's beta, it's free. Just write down you email on our awful web site (sorry we are C++ developpers not web designers) and you'll get a free beta version : beta | www.springbeats.com
This might do it for you.
Otherwise, you've got plenty of commercial options, starting with reaper (shit I think I'm advertising for competition
) which is very good and very cheap.