GnuCash is personal and small-business financial-accounting software, freely licensed under the GNU GPL and available for GNU/Linux, BSD, Solaris, Mac OS X and Microsoft Windows.
Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing.
Invoice Ninja is a free, open-source solution for invoicing and billing customers. With Invoice Ninja, you can easily build and send beautiful invoices from any device that has access to the web.
InvoicePlane is a self-hosted open source application for managing your quotes, invoices, clients and payments. The application comes with various options, templates and settings to customize your invoicing experience.
From invoicing to expense tracking to accounting, Akaunting has all the tools you need to manage your money online, for free.
Manager is free accounting software for Windows, Mac and Linux.
It features an intuitive and innovative user interface with modules such as cashbook, invoicing, receivables, payables, taxes and comprehensive financial reports.
The desktop edition is free while the cloud edition.
BILLmanager is a highly customizable billing software for hosting and IaaS business with the full scope of functionality for billing, service provisioning, client management & support.This app doesn't have any features at the moment.