A custom feature-flag system for my current firm. It lets them roll out things unattended ("add 2% of users every day until it's fully on"), gated in ways that

A custom feature-flag system for my current firm. It lets them roll out things unattended ("add 2% of users every day until it's fully on"), gated in ways that fit their business structure. We're in a space(payments) where deployment is a tightly managed compliance thing, and people get very touchy about specific assumed behaviour. A lot of support inquiries are literally "you just fixed things and got fully certified to be compliant with a third-party service? But we LIKED the old behaviour!" Feature flags have lowered the tension. We can put sensitive customers into a penalty box until they're ready to use the updated feature, and if something blows up too badly, we just entirely deactivate the change until it can be properly fixed, with a few clicks. Virtually any significant project has one or more flags, and it's ,mostly been a "alternate Fridays are for side projects" sort of task.