Over at Church Crunch (kind of a tech crunch for churches) they have a post about the history of WordPress. This graphic also shows the most downloaded plugins. These include:

1. All in one SEO pack 5.9 million downloads

2. Google XML Sitemaps 4.3 million downloads

3.  Akismet 3.4 million downloads

4. Contact Form 7 2.3 million downloads

5. Nextgen Gallery 2.6 million downloads

6. WordPress.com stats 1.8 million downloads

7. WP Supercahce 1.4 million downloads

8. WP Touch iPhone Theme 1.05 million downloads

9. Google Analyticator 1.03 million downloads

As of April 4th 2010, WordPress 3.0 has been downloaded over 15 milion times (add 2 downloads per second to get to the current download total) , and 8.5% of the Internet is using WordPress.