When you get involved in web hosting, there is a good chance you will need invoicing software. There are a few good hosting billing managers if you have your own web hosting company, but if you are doing other work, such as web development or server maintenance, you may want to use an invoicing system. The following are some free and open source invoicing apps that are easy to use and can run on your own website.
Simple Invoices – A cross-platform web app, Simple Invoices allows you to track your finances, manage customers, export invoices to PDF, use payment gateways such as PayPal, and manage inventory.
siwapp – This easy-to-use invoicing system includes support for automatically generated invoices, PDF exports, and multiple currencies.
Ofuz – Ofuz provides a cloud service but also offers its software as an open source app that you can install on your own server. Features include time tracking, task organization, collaboration, emailing and web forms, online payment support, and more.
MyClientBase – This community-drive app allows you to collect payments online, track invoices, create and edit templates, configure taxes, and use it in multiple languages.
Amberdms – Much more than simple invoicing, Amberdms is a complete accounting, billing, and service management application. You can keep time, export to CSV and PDF, perform regular service billing, and integrate third-party add-ons using the SOAP API.
All of these applications are freely available under various free software licenses, and because their source code is open, you can modify them to suit your needs.