In the recent article from San Jose Mercury News Scott Kirsner writes

... many of us have become obsessed with audience metrics of our own,    seeking a tally of how many people we're reaching as something of a touchstone - an indicator of our influence, popularity, or coolness.

He quotes Scott Beale, blogger of the Laughing Squid reporting that drop in visitor count can lead to depression. Similar opinion is shared by Daniel Klass, podcaster of The Bitterest Pill.

Evolution of todays free and very flexible metrics systems has lead to an obsession over the metrics. Bloggers are tracking where their readers are coming from, which browsers and operating systems they are using, and which are the primary referrers to their point of destination. While in the early era, it was all about the visitor counter, nowadays analytics software is very sophisticated and allows plenty of tweaking and narrowing down from one category to another.

Primary metrics services are
Google's Analytics
SiteMeter
StatCounter
Extremetracking

These are just the ones I used/heard of. But doing a simple Google Search gave me a whole bunch of other services :

FuseStats
ShinyStat
OneStatFree