I noticed that one of our S3 buckets had high data transfer costs, a bucket that our app downloads HTML+JS assets from when we push out a new release. I downloaded the "directory" of files for our latest release and saw it was mostly node_modules. I checked the code and confirmed that, yes, if this file exists in the bucket then it'll be downloaded by the user. I wrote a quick Python script to list out each directory that had this problem, and a quick Slack message to the appropriate team later, we discovered the specific commit that was the cause, a change to our CI that inadvertently uploaded that directory when we wanted to ignore it.
A few months later, I checked the billing metrics, the effect was an avg of $12,500 reduction in cost for this bucket, or around $150k per year, or 4% of our bill. Not bad for one hour of work. Over the course of a quarter I reduced our bill by over $1m, or around 30% of our bill.
I might write a blog post explaining how to go about something like that. A lot of people are not familiar with tools like Trusted Advisor which can easily tell you if you have, for example, unused EC2 instances that can be terminated.