Toggl

If you are looking for a way to track how much time you spend on different tasks for multiple Clients and Projects then Toggl could be a great tool to take a look at.  It has both a free and paid version and the free version has enough features for anyone.  You can setup multiple different Clients and Projects and start tracking your time within minutes and be using that data to invoice for your time.  With apps available on all devices as well there is no reason why you cannot track your time where ever you are.

Check it out: toggl.com