Thanks to Robert from the forums he convinced me to implement a feature for him. Well, actually he bribed me to do it. ;-)
Nevertheless, now that I implemented the feature, I kinda like it. The enhancement to the Freetag plugin (version 2.7, should be available via Spartacus now) allows you to enter a list of comma seperated keywords for each tag you have available on your blog.
Whenever you save an article now, the plugin will analyze the content of your entry. For each keyword that you entered and that is found in the article, the corresponding tag will be auotmatically assigned to your entry (taking care that no duplicate tags happen).
So, if you have the Tag "PHP" you could enter keywords like "Serendipity,php,s9y,phpbb,xss,sesser". When you now create an entry where you use the keyword "Serendipity", the freetag plugin will automatically assign the tag "PHP" to this entry.
Thus, especially if you have a low count of tags you can save a lot of time by assigning meaningful keywords to your tags. Beware that if you enter a lot of keywords for a lot of tags, that this might slow down saving an entry. This happens because a list of EVERY available keyword needs to be compiled and matched against your saved article to be able to see which keywords were used.