Simple Task Timer - measuring time spent on tasks and projects
EditByBSEditor: Simple Task Timer is a free multiplatform (Windows, Linux, Mac OS X) desktop application for tracking time spent on tasks, projects.

With Simple Task Timer you can bill you client without guessing about time spent on his project. Also by using data of previous projects you can better estimate time for new, Similar, project.

There are limited set of features:

creating projects
adding tasks to projects
start and stop buttons
reporting with export to CSV file or MS Excel XML format you can free download Simple Task Timer 0.3.1 now.