I’d like to suggest Fanurio (http://www.fanuriotimetracking.com) which is a time tracking and billing software application, highly appreciated by freelancers for its intuitive interface, flexible timer and ability to produce detailed invoices.
Fanurio is not free but it has a free trial. It costs 59 USD (which means about $5/month for the first year) and you can use it for life.
I know that there are many freelancers who prefer to use a desktop application in order to keep their data on their own computers. Since Fanurio is a desktop application, it integrates very well with the platforms it runs on. On OS X, you can easily access timers from the the menu bar and from the dock icon menu. On Windows, you can start a new timer or control the active timer from the thumbnail toolbar or from the tray icon. You can also use global hotkeys to control the timers from within any running application.
Fanurio is used by many freelancers not just to track time but to bill their clients as well. Fanurio can export invoices to HTML, PDF, Microsoft Word 2007, OpenOffice OpenDocument and other formats so they can be printed or e-mailed. Invoice templates can be created manually, with a visual editor (Adobe Dreamweaver, Microsoft Word or OpenOffice) or with the built-in template editor.
I hope this helps.