Toggle Function Keys for OS X (10.11)
This is a modified version of an AppleScript from Macsteps, updated to support OS X 10.11 and to put up a system notification so you know what state the keys are in.
A signed app-ified version of this script is coming soon; in the meantime, to get an easily-run binary, open the script file and choose "File -> Export" with File Format of "App."
Note that this will only work in English at the moment due to its reliance on GUI scripting.
This (in theory) should work on any OS X 10.8+ English Mac - I've only tested the latest version against OS X 10.11, though, so feel free to fork/PR if you run into any issues on other versions of OS X.