This is a placeholder for me to do development, testing and personal use. The intent is that no applications will be released beyond my own use using the credentials associated with this placeholder.
2019-09-04: Yes, this placeholder is still in use. Off the top of my head, the write tokens enabled by this placeholder have been/are being used for development and testing of FIRE. I'm vaguely recalling that I've used the write tokens permitted by this placeholder on at least another script or two over the course of this placeholder's existence, but I'm not remembering which one(s).
While a placeholder post isn't necessary for an API key, the API key associated with this post is used on several different scripts while I've worked on them, including, but not limited to, the Unclosed Request Review Script, MagicTag2, AutoReviewComments, and more. I edit and contribute to more than 50 different scripts associated with Stack Exchange. I don't recall all of the ones where I've used the API key associated with this post. Generally, I've done so in order to insulate the users of those scripts from any repercussions from changes I make in testing/development (i.e. if something goes very wrong and SE needed to shut off access for the API key used in a development version of a script, doing so would not affect the users of the released version).