Wordpress.com Stats Plugin
Wordpress.com is the free-hosting way to get a WordPress blog. While most serious bloggers install WordPress on their own domain, Wordpress.com is a pretty full-featured solution for casual bloggers. One feature that even solo WordPress users have been jealous of is Wordpress.com's "Stats" system, which is a set of blogging-oriented traffic metrics. The Wordpress.com Stats plugin was written to extend this functionality to self-hosted WordPress blogs.
Wordpress.com Stats is one of the fastest stats systems around because everything is computed on Automattic's servers (those are the people who created WordPress). You might have noticed, if you're very observant, a little, teeny-tiny smiley face in the footer of some blogs you've visited. In fact, that smiley face is the Wordpress.com Stats plugin's way of counting visits, and blog administrators can hide it via CSS if they don't want people to see it.
For fast, free, real-time collection of popularity data, pageviews, traffic sources and more, Wordpress.com Stats comes highly recommended for a WordPress traffic monitoring plugin.
Image credit: Screenshot © Automattic.