Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
MachPanel - MachPanel is a Multi-Cloud Service Orchestration & Delivery Platform.
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Blesta - Blesta is professional client management, billing, and support software.