You might like TimeStamp by Syntap.
http://www.syntap.com - it's free. You save 1 project per client. What I like to do is save for each billing cycle, start a new one each time and save by clientname_date. Also, you can print the timestamp report for your records, and create a PDF of the report to submit to your clients with your invoice.
TimeStamp does not have tons of bells and whistles but if you want a basic timer that will allow you to create quick reports and time your work - this is the one I recommend.
I have used other timers through the years, but always go back to trusty ole' TimeStamp!
Nancy
virtualgalfriday.com