I wrote my very first plugin for WordPress.
The only thing it does is to append a string to the name of the author of a comment if the email address of the author contains a certain string. I was using the get_comment_author filter hook of WP. It is activated whenever the author of the commentator gets out of the database.
More information about plugins and hooks in wordpress: http://codex.wordpress.org/Plugin_API
A list of the hooks in wordpress with displayed source code of the usage: http://adambrown.info/p/wp_hooks/hook