EyesOfNetwork is free and is designed to allow easy implementation of a library process (ITIL). It simply shows information, raises alerts when there is trouble, helps the team to solve it, and very easily provides SLA reports in an editable format.
It allows you to create invoices very simply in a php web site.
The Koozali SME Server is a simple and robust server gateway for SOHO
The Koozali SME Server is the open source community side of the original e-smith server. SME Server consists of a modified Red Hat / CentOS Linux installation, together with a number of server applications as well as the server management web panel. Its strengths are its ease to use for anyone able to use a web interface, and its robustness, giving the ability to recover its initial state even after any upgrade. The server is designed for Small Offices and Home Offices to act as a gateway with a firewall web and email services, but fits also home user needs or even the needs as a simple and robust server only in the cloud to host your website projects and emails.