I've been using Google Analytics on this website since the beginning.
The dashboard (the main page) allows you to take multiple views of your data and drill down if necessary. It includes a drop-down to select the date range (flash I believe) and also to compare up to 2 metrics at once.
I spend an inordinate amount of time on the site. Any new visitors? How many so far today? What pages are being viewed?
Of course it's not perfect. I'm not sure, other than requiring all users to login, how you can determine the number of unique visitors.
Far and away my favorite view is the one that shows you the distribution by continent, sub continent region, country/territory, or city. Recently they added the ability to drill down as well.
It's definitely cool to see the different people from all over the world who have viewed the site.
Wait, how could I forget? The cost? Free!
Doesn't get any better than that.