When I switched from Drupal to WordPress as the software that runs my blog, one of the things I missed was tag autocompletion while writing posts. Drupal has a great freetext tag field, including an AJAX utility that automatically checks with the server for tags that match what you've started to type in the tags field.
And now, WordPress has that too! With the Simple Tags plugin. It has some other features, too (but I haven't tried them yet) but the tag autocompletion is killer. Why isn't that included in a stock WP install?
Here's what the autocompletion offered as I entered the tags for this post: