who's.amung.us is a nifty tool that tells you who's on your site at any given time of day. It also tracks where they came from and lets you know which of your pages are the most popular.
You don't even have to register to use the service. Just grab the code off the index page and insert it into your sidebar or footer. Placing the code for the counter within a post allows you to track readership for that particular post. Click "Customize Your Widget" and alter the counter's background and font colors to suit your blog. In addition to the "classic" counter, there are several other widgets to choose from. There's also a "Maps" widget, which shows current visitors on a world map.
When you or a reader clicks on the counter (try it below), you'll be directed to a stats page that includes "current pages viewed on your site, a world map of visitors, and historical graphs of your user counts." If you use Firefox, you can download an add-on that lets you keep track of who's visiting your site from your browser, even if you're browsing somewhere else.
If you've got good traffic, announcing your readership to the world can be a good thing. However, Readers who visit your site to see that they're the only one reading may feel like they've walked into an empty room. Still, tracking readership patterns such as when you get the most visitors allows you to time your posts to reach as many people as possible (see the article below for more on this). While you can get the same information from other stat-tracking services (like Google Analytics) who's.amung.us is easy to use and, in my opinion, more user-friendly. It's also pretty fun.
Related BlogCoach Post: Get the Most Out of Your Peak Traffic Days