OpenBravo POS and Unicenta
OpenBravo is a longtime popular open source tool for managing the flow of inventory in businesses, aka ERP. The constellation of tools now includes a Point Of Sale companion for running cash registers at stores and restaurants. You add your inventory, and then the software keeps track of what is sold when the customers check out.
OpenBravo POS is also attracting some forks like one from Unicenta, an option that continues the open source branch of the project while offering support. They're cleaning up the older code, adding some fresher visual designs, and releasing it under the GPL 3.0 license.