WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
Toggl - Toggl is an online time tracking tool. It features 1-click time tracking and helps you see where your time goes. Free and paid versions are available.
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.
Clockify - Simple and free time tracker. Perfect for small and mid-sized businesses as well as freelancers. Unlimited projects and users, unlimited productivity. Get all the premium functionalities, completely free.
Blesta - Blesta is professional client management, billing, and support software.
Cattr - Cattr is a free open-source self-hosted time tracker with desktop clients for windows, mac, linux.