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.

Office Timesheets - Professional Services Automation

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

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

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