I started using the free time management software from RescueTime.com a few months ago and have found that it helps keep me from venturing too far off track during the work day. I had previously been using a small app built by the Software Jedi (my hero! I still use it for making my time cards), but it is buggy and lacks any real analysis features (still not bad for being built in one day).
I was planning on building a bunch of macros and pivot tables in Excel for it when I ran across RescueTime, which is free for personal use and actively being developed. It keeps track of all the websites and applications you use on your computer and sends this data to their servers to let you tag each item as productive or not-productive. From all this tagging you get a bunch of pretty pictures saying how much time you spend working and how much you waste reading web comics or Slashdot. They even have a flash widget you can add to a blog. Haven't tried the Team edition yet, but it has definitely helped me keep from wasting too much time.
One person's error is another person's data.