Node.js and NPM
While the previous example is fun, no more reaching for the remote, it isn't super practical. You wouldn't bill a client for creating something like this.
Google Apps Script
They have an example showing how to send emails from spreadsheet data. This gets me thinking about automating client reporting…
Automate Your QA Process
After I run WordPress updates (core, plugins, and themes) on a site’s staging environment, I use the app to do a pixel-to-pixel visual check to make sure nothing has changed. This is something I used to do manually, which was very time consuming, and I still ended up missing things occasionally.