Tryton - business application collection and framework
Tryton is a business GUI application framework and a set of applications of that type. It supports invoicing, tasks, calendar and much more. This framework works in a client-server manner and uses PostgreSQL database, PyGTK+ GUI toolkit and is available for main operating systems (the client part).
I've installed pre-made packages from Linux Mint / Ubuntu repository and used one of Tryton test servers to test the client application:The application runs fine, not problems with stability. In Tryton there are many finished applications that can be used: I can't rate how much functionality they provide, but they do look interesting. And it's Open Source software.