Inline Tag Thing allows you to add and remove tags on posts, inline, using magical AJAX powers.
There are two advanced options. To change these options, you'll need to set a couple of flags in the plugin file.
Changing $automagicEmbed to false will stop automagically including the inline tag adder at the end of your content, which allows you to use the ITT_ShowWidget() template tag to display the inline tag adder in a location of your choosing.
Changing $showExistingTags to true will add a dropdown list containing the tags which are currently in use which you can add by choosing a tag and clicking the + button.
Also, there's a CSS class, itt_tagBox which you can use to override the display of the inline tag box.