Keeping track of time

Do you use software to keep track of time spent on various parts of your 
projects?  I'm familiar with one shareware product called Time Slips.  It's a 
PC product that I use at work.  It provides nice data reports broken down by 
client, by date, etc., etc.

I'm about to go freelance and want to poll the forum to see if there are any 
preferred alternatives.  I'm interested in both the PC and MAC.

Thanks
++Dan

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