Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
TimeSite Pro - TimeSite Pro is a fully configurable, cloud-based time tracking, expense & invoicing solution.
GoodGantt - The best Gantt chart for Trello®
Minutes - Minutes - Automatic efficient time tracking and time accounting software for lawfirms
Paws for Trello - The best way to use Trello on your desktop
OutlookTime - OutlookTime is a time tracking system that turns Outlook into a timesheet system.