OpenBravoPOS is one you could use, but I'm not sure how well it would work across several locations. I think there'd be a nightmare to some degree getting all of the data put together. It's supposed to be more of a front end for OpenBravo (and it used to be its own app called TinaPOS back when I was helping with documentation) and I don't think it can stand on its own.
OpenBravo requires a lot of cash to actually start using (it looked like that to me anyway when I was researching ERPs) and there was something weird about OpenERP too that I can't remember.
Tryton (tryton.org) is an open source ERP you might want to check out; it seems to be the only one out there though that is truly open source in name as well as spirit. I'm not sure yet what point of sale looks like because I haven't gotten too far into it. Family stuff out this on the way back burner for me.