Even though the website looks a bit outdated, this program is still impressive as it has its own GUI and even compiled with Java 7. I know it is currently only written for the UK market but having an internationalised version would be of interest as well. At lot of ERP-systems on Linux are web-based but Jalia is written in Java. Three stars only as it apperently still relies on Telnet and three stars (meaning average) that I couldn't find a list of partners at first sight.
Fast and simple.