A simple task: list all the users on a site, optionally filtering by role or by status. Difficulty: using drush.
I had searched around a bit for this functionality and my Google-fu had failed me, so I decided to build off of work that already had been done and write a drush user-list command. (The internal monologue went something like this: "Is this a thing? It doesn't look like this is a thing. This should be a thing. Why is this not a thing? Let's make this a thing.")