Personal site.
A command-line tool I run on my own machine to read, search and draft mail across several of my own Gmail accounts. Each account is pinned to its own local credential store, so a command issued for one mailbox cannot reach another.
It runs entirely locally against the Gmail API. There is no server, no hosted component, and no account other than mine involved. See the privacy policy for what it does and does not do with data, and the terms of use.