We came across a free open source Point of Sale (PoS) application called Tina/Libre POS a few years ago when looking for a solution for a leading leisure industry client in Europe.  POS was a key requirement for them. 
Their leisure centres included bars, restaurants, sport shop outlets and spas as well as gym, tennis and other facilities.  Our remit was to find a flexible POS system able to integrate and cater for the needs of hospitality, leisure and retail.

Tina/Libre POS was closely considered for the project and even though it didn't do everything wanted at the time, the POS from the Club Membership system supplier did even less by the way, it certainly gave enough flexibility straight out-of-the-box to immediately overcome several immediate challenges.  With further development it could have met the objectives and gone on to be a key, fully integrated Line Of Business application and the the corner-stone of the group's customer facing retail activity.
However, the decision was made not to go ahead with Tina/Libre POS as support was considered a key risk issue, especially for a lesiure operation that runs 18+ hours a day x 7 days a week, where continuity at point of sale outweighed the benefits of acquiring a non-commercially supported POS which met most of the business requirements.

We promised, one day, we would revisit the Tina/Libre POS project and do something more with it.  We have; it's called uniCenta oPOS.

In the last few years the Tina/Libre POS project became Openbravo POS and it has evolved into a very usable, mature and robust application.  Openbravo has integrated it into their successful Openbravo ERP product.
Openbravo POS is still free open source just like uniCenta oPOS and with almost 10,000 Openbravo POS installations per month (according to Openbravo web site) it is enjoying world-wide success and is possibly the most downloaded, installed and used Point Of Sale software available today.