Consider Mycroft
There are a number of open source voice assistants available now, and given the popularity of the commercial ones and how intrusive they are privacy wise, I think it might be time to add this as a category and list at least one or two. Issue #898 has been outstanding for a while now, but I'd like to see it take it's original form of noting the proprietary things to be avoided (Google Now / Google Assistant, Apple Siri, Amazon Echo / Alexa, Microsoft Cortana, Samsung Bixby).
By reputation I know the most about Mycroft, and this issue is to propose adding it. There are others to look into (Lucida, Hey Athena, Blather, Dragonfire, Kalliope) but I would suggest if anybody wants to see them added that each should be proposed in a separate issue one per project for review.
Mycroft can be run on dedicated hardware, on a Linux desktop, an a Raspberry Pi, on Android, or baked into something else. All the sources for various platforms are on the organization's Github. The core is written in Python and Apache licensed. Some of the front end clients are GPL, libraries to access it are LGPL, etc. Basically everything seems to be available to be able to run assistants that don't call out to the web or turn a microphone over to some corporate overlord.