Free :uniCenta oPOS is Free Open Source POS

uniCenta oPOS is a proven, reliable Touchscreen Point Of Sale application that meets the needs of a wide-range of businesses.  It's a great POS for for all kinds of operators from Amusement Parks and Bars through to Yacht Chandlers and Zoo shops!
Highly configurable and reliable uniCenta oPOS runs on Windows, Linux and Mac OSX and can scale from a single PC through to tens of POS terminals spread across different locations.
uniCenta oPOS started life as a branch of the amazing, but now defunct, Openbravo POS open source project back in 2010.  Since then we've extened it, improved it and added a load more functionality.

"Openbravo POS was a popular, open source way to run point-of-sale devices such as cash registers or checkout computers. The Openbravo POS project stalled in 2011, but the source code lives on in a very powerful and beautifully redone fork, uniCenta oPOS. The folks behind the new project have gussied it up with a new look (along with 38 different skins), provided commercial support for customers who needed it, and released the source code as GPLv3 to prevent it from being resold as a service. Everything from automatically figuring sales tax to inventory management, scale integration, and employee control is included."