Is most of your work done on your computer? Would it be helpful to know how much time you spend using the various software installed on your computer? With an easy way to tag usage to a particular project or client? If so, the freeware ManicTime may be just what you need.
Features (From the web site)
ManicTime automatically collects data on your computer usage. It records active and away time, as well as which applications you used and for how long you used them.