I made an Elasticsearch querying tool that lets you construct multiple dependent queries.

If I hadn't taken an Algorithms class, I wouldn't have thought to use a directed acyclic graph, and to topologically sort the different queries so that they are resolved in order.

That was actually the first time I'd used something from a theory class on the job! It was pretty exciting, though probably mundane for most engineers :).

That's actually a great question and I don't think I can really answer it yet since I've only done internships and I'm set to start my first full-time job in January. I find some satisfaction in trying to optimize the small problems I've faced so far but it's mostly been bug fixes and glue code. I like the answer Feuilles_Mortes gave, hopefully I'll find some useful scenarios in the future.