I've created a new demo showing how to start writing plugins in IntelliJ IDEA. You can see it at http://www.jetbrains.com/idea/training/demos/google_search.html
It is mainly based on the article "The Basics of Plugin Development for IntelliJ IDEA" by A. Efimov (you can find it here: http://www.jetbrains.com/idea/documentation/howto_03.html).
But instead of the "HelloWorld" example it shows creation of the plugin that searches the word under the caret.
We'd like to provide plugin developers with more information, tutorials, articles, etc. We are looking for experienced IntelliJ plugin developers who want to write articles on this topic. If you are one of them, contact Alex Tkachman to discuss the terms of the agreement.