Wordpress is one of the friendliest environments to write extensions for. I have done a few , but for now you will have to be satisfied with just this one

Keyword Link Plugin

This Wordpress 2.3+ plugin allows you to automatically link certain keywords in your Wordpress blog. So if you would like to link every mention of the word “Pizza” to your “Pizza Speciality” page this is the plugin for you. Another use would be for a tourist blog, where you would like every mention of a city name to link to the page describing the city.