Anuko Time Tracker is an open source, free, web-based work time tracking system, written in PHP. It is simple and very easy to use. It allows you to create user logins and organize them in teams, create and modify projects and tasks, input work time, generate reports and invoices and send them via e-mail. It has three time tracking modes: time only, time by project, and time by project and task. The following plugins allow for extended functionality: charts - for visual diagrams of the time spent on projects and tasks, clients - for allocating time to clients, invoices - for invoicing clients, and custom fields - for an additional text or a dropdown custom field to include with each time entry.
Reports, invoices for billing. This software is popular with small companies who need to do basic time tracking, reports, and billing (invoices).
Anuko Time Tracker is a simple and easy to use web-based time tracker. At the same time, it is a powerful solution, see the reports feature for an example of this. And don't forget, it's open source.