VOGSY - Professional service automation tool built on Google G Suite

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.

HarmonyPSA - HarmonyPSA - Professional Services Automation (PSA) for IT Businesses

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Timewatch - Timewatch offers time tracking, expense management, resource scheduling and professional services automation solutions.

Task Coach - Task Coach is a simple open source todo manager to keep track of personal tasks and todo lists.