[pkaeding]

This site has a pretty good survey of the open-source options in various languages: http://featureflags.io/feature-flags/ (you have to pick a language from the menu to get to the page specific to that language, as the page I linked to is more of an overview of the concept, and there doesn't seem to be one page that lists them all).

[RileyJames]

FeatureFlags.io is owned and operated by the CTO of LaunchDarkly. It’s not a survey, it’s an infomercial / microsite for LaunchDarkly. It states “contributed by LaunchDarkly” at the bottom, but considering it’s owned by them that doesn’t seem to adequately sum up the situation. Not sure why they wouldn’t just call it as it is.