Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
VOGSY - Professional service automation tool built on Google G Suite
Harvest - Simple time tracking, fast online invoicing, and powerful reporting software. Simplify employee timesheets and billing. Get started for free.
HarmonyPSA - HarmonyPSA - Professional Services Automation (PSA) for IT Businesses
GnuCash - A personal and small-business financial-accounting software, licensed under GNU/GPL and available for Linux, Windows, Mac OS X, BSD, and Solaris.
Timewatch - Timewatch offers time tracking, expense management, resource scheduling and professional services automation solutions.