Lemon is an opensource point of sale for linux, and other unix. 
It is a general POS, not targeted to a specific business.

It has been developed for ease of use and customization, and to support
barcode scanners and ticket printers. At this point, it has been tested
with a parallel port ticket printer (Star Micronics, SP500) and a barcode 
reader (datalogic DLC6065-M0).

More information on 

http://lemonpos.sourceforge.net/
http://lemonpos.wordpress.com/

