WordPress.org

1. Download the archive file and uncompress it.
2. Choose the appropriate version, and plop it in wp-content/plugins
3. Enable in WordPress by visiting the "Plugin" menu and activating it.

This file contains two plugins, one for use with Wordpress 2.3 and one for use with WordPress 2.0.x-2.2.x (labeled as "wp22.") Don't use the 2.3 version on 2.0-2.2 installations - it's not catastrophic but it won't work at all. (Same goes for the 2.2 version on a 2.3 installation.)