[ashbrahma]

A gmail application that would parse through your email (old and new) and collect all contact information without having to do this manually. Especially helpful if this can be done for older backed up email.

[ww520]

This can be useful. Connecting to Pop3/Imap to collect the data is not too bad.

[plumeria]

This looks similar to what you asked for: https://www.evercontact.com/. Not open source, though.

[mynameislegion]

https://github.com/aperezdc/notmuch-addrlookup-c