WordPress.org

Wordpress.com Popular Posts can be installed easily:

1. Download and install Wordpress.com Stats plugin.
2. Download Wordpress.com Popular Posts .zip archive
3. Extract the files in the .zip archive, and upload them (including subfolders) to your /wp-content/plugins/ directory.
4. Activate the plugin through the 'Plugins' menu in WordPress
5. Inside the WordPress admin, go to Design > Widgets, and add the 'Popular Posts' widget where you want, then save the changes.
6. If you want, you can customize some settings for the widget, in that page.