Here is another small and simple but yet powerful plugin.
With seKeywords you can customize content on your pages or posts depending on the visitors referring search engine and keywords. This means that you can make sure that the visitor will find what he/she has been looking for. You could change the prominent text like page titles to include the keywords they have been searching for.
There are various ways to customize when what content should be shown. Currently only Google, Yahoo and Bing are supported.
There is a small guide that explains the options for this plugin.
You can download the plugin here.