I write a lot of internal tools and such, so that is where it's useful (nothing really public facing)

Maybe not "apps" per se, but my three main use cases in my daily chores are:

* A ruby script that deploys our application

* A ruby script that cleans up the repository after deploy

* A terminal-based git script that runs through all modified files, shows me the diff and allows me to either [c]ommit, [d]iscard changes, [s]kip or [q] before I commit.

That last script is a prime candidate to use those tty-* gems.

I have a bunch of scientific data pipelines and have found rake to be an excellent tool for these. Same for a bunch of "housekeeping" scripts.

Rake is seriously underrated I believe. It allows for dependencies and rules, figures out which steps are up-to-date and skips those, allows easy parallelisation etc. Much like make, but with the familiar and sane (ye beautiful) syntax of ruby and its extensive ecosystem.