TopTracker - Beautiful, free time tracking

WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.

timeBuzzer - timeBuzzer enables freelancers and project-driven teams to track their time at the touch of a button, understand where their time goes and create reports with just a few clicks.

HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.

Cattr - Cattr is a free open-source self-hosted time tracker with desktop clients for windows, mac, linux.

Blesta - Blesta is professional client management, billing, and support software.