qtill

Qtill is a Qt based application that allows you to set up a sales terminal without specialized equipment, but using only regular PCs, notebooks or tablets. It is mainly intended for temporary uses in tax exempt scenarios as it is not a complete solution.
It does not currently support credit/debit card readers, nor specialized receipt printers.
Qtill works with or without touch-screen, and can optionally print a receipt with a regular printer.
It has basic inventory and multiple terminal support.
It is currently optimized for restaurants and similar activities.
Feel free to require other features or to help implementing them.