There are a few open-source feature flag libraries out there, but what makes our service better (IMO) is mostly the cross-platform consistency (so a user will get the same value of a flag across different services, mobile, etc) and the ease with which non-technical people (eg product managers, etc) can adjust the targeting.

Of course, you should evaluate the options for your situation. For many use cases, an open source library will solve the problem just fine. For larger projects/teams, something like LaunchDarkly will be a better fit.