Akismet is, by far, the most popular anti-spam plug-in for WordPress. (It comes bundled with the download, so that gets it market share. But, it's also very, very good.) It comes with a counter that can be put into a WordPress theme. It's attractive, but its light blue color may not integrate well into a given theme.
I went digging around in the source code, and found the line that actually pulls the count from the database. Using this parameter, I was able to integrate a spam count into the sidebar that has a look consistent with the rest of the site.
Here's the code that's in use on the theme on this site.
<li id="spamstats"> <h2><?php _e('Akismet-Eaten Spam:'); ?></h2> <ul> <li><a href="//akismet.com"><?php echo(number_format(get_option("akismet_spam_count"))); ?> and counting...</a></li> </ul> </li>
Of course, line 5 is the important one - that's how to get the number, formatted for whatever locale the server is set up for. (On my personal blog, the number is up over 1,400!)