Open source point of sales app

[Becker, Rob]

Does anyone out there have experience with any open source point of
sales applications?  I'm helping my parents with the technical side of
opening a restaurant and I am trying to find a good POS app for them.  I
would prefer it be open source or at least able to run on Linux.  It
would be very helpful if it could be easily integrated with Quickbooks.
Does anyone out there have any experience with this kind of endeavor?
Any links, advice, hints, tips to offer?  All help greatly appreciated.
Thanks.
Rob