There's a bunch of feature toggles libraries out there for pretty much every web language you can think of. Doing it statically is pretty easy, you can rewrite a similar library in a day or two. One Ruby[0] and Python[1] example below, but there are tons.
What you don't get out of those libraries alone is a couple things. One is a metrics pipeline if you want to actually A/B test and gather metrics on the success of different features. That's a hard problem because A/B metrics collection pipelines are something pretty much something everybody reinvents from scratch right now to fit their combination of language/logging tool/cloud host/data warehouse/BI tools/whatever.
The other big one is the managed configuration deployment - a UI that you can toggle features on and translate those to appropriately flipped toggles in all of your clients (mobile, web backend, web frontend, whatever). That's a pretty big task to do and requires a hosted service one way or another. Flipper (below) looks like it integrates into a rails service and so has a UI, which is kinda neat (first time I've noticed an OSS solution that does that part - looks like theres a number of options that do this now), but you'd still need to reinvent tooling for mobile apps and such with it.
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).
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.
What you don't get out of those libraries alone is a couple things. One is a metrics pipeline if you want to actually A/B test and gather metrics on the success of different features. That's a hard problem because A/B metrics collection pipelines are something pretty much something everybody reinvents from scratch right now to fit their combination of language/logging tool/cloud host/data warehouse/BI tools/whatever.
The other big one is the managed configuration deployment - a UI that you can toggle features on and translate those to appropriately flipped toggles in all of your clients (mobile, web backend, web frontend, whatever). That's a pretty big task to do and requires a hosted service one way or another. Flipper (below) looks like it integrates into a rails service and so has a UI, which is kinda neat (first time I've noticed an OSS solution that does that part - looks like theres a number of options that do this now), but you'd still need to reinvent tooling for mobile apps and such with it.
[0]: https://github.com/jnunemaker/flipper [1]: https://github.com/disqus/gutter