I must confess I haven't used it yet, but snips.ai [1] is in my list of potential raspberry pi projects and claims to be open sourcing soon. On their FAQ page [2] you can find the reason why I have it in my list:
"Snips, on the other hand, runs completely on your device, with nothing being sent to the cloud. This means it guarantees your Privacy, works offline and doesn't have variable costs!"
Unfortunately this leads me to wonder whether or not the service will remain free, but for the moment it is "free for makers and for building prototypes". Commercial costs apply.
[2] https://github.com/snipsco/snips-platform-documentation/wiki...