Workaholic

Workaholic is a location-aware app which allows you to clock in/out and track hours worked for your jobs/projects.

Set-up your work locations and automatically start logging your hours. You may also manually log hours by using a single start/end push-button or a convenient app widget.
Export your timesheet to Google Docs or as a CSV for further use.

Supports different billing periods - Daily, Weekly, Monthly and Bi-Weekly.