Compared to compressors and limiters, I think expanders might be the "forgotten" dynamics processor. Maybe that's because the most common application is downward expansion, i.e., below a particular threshold, the output level falls off at a faster rate than the input level. This helps reduce tape hiss, noise from guitar pickups, low-level hum, etc. But in today's studios, noise levels are pretty low, so expanders aren't as relevant as they were in the days when hiss of all kinds (tape, preamps) was common.
However, lately I've been playing with the expander in Studio One, which can also do upward expansion. With this, you set a high threshold, and expand upward from there so that the output increases at a faster rate than the input. What got me into using this was a drum loop that had a lot of room sound. It sounded cool, but the drum attacks got kind of lost in the process. By setting the threshold at -3.0, and the ratio to 1:2, the peaks above -3 became more prominent - problem solved.
I've also used this to accent the string attack with acoustic, and make the overall sound a little more percussive. This allows mixing the guitar further down to make room for other instruments, but the strumming itself is still able to drive the song.
Any other expander fans out there?