Apps that provide notifications of things happening.
Sometimes SE's limited notification facilities just don't cut it. Sometimes you want to know about other things - new questions, active questions, comments by Jon Skeet, or just something out of the narcissistic ordinary. If you have an app that does that, or are looking for one, this tag is the place to be.
Note that it doesn't really matter what sort of notifications your app provides - chrome-extensions notifications, systemtray notifications, audio notifications, psychic notifications... - use this tag anyway!