Harvest API

From their site: Harvest provides two API interfaces, serving two distinct roles. If you need to access and manipulate your daily timesheet the Timetracking API fits the bill. Notable uses are the widgets we provide for PC and Mac as well as other third party timesheet software integrations. If you need to access and edit your projects, clients, users and tasks the Extended REST API is your choice. You can use this to mass import your existing projects setup, add users and generally integrate with your existing back-office setup.