Steward
Steward is a self-hosted assistant that runs on hardware Evan Williams owns. It is not a public service: only Evan and members of his household use it, with their own Google accounts.
What it does with Google
When you sign in with Google, Steward can read and organize your Gmail, draft and send email you ask it to, read and manage your Google Calendar events, and find and read files in your Google Drive. It uses this to answer your questions, keep the family calendar up to date, and handle the email and scheduling tasks you give it. Anything it sends or changes on your behalf goes through an approval step first.