The new Google Contacts: Bringing everyone together
gmailblog.blogspot.com
gmailblog.blogspot.com
Inbox uses MD heavily, but it was brand new and built from the ground up using it. As far as I know, Contacts is the first piece of the website puzzle to implement it after the fact.
EDIT: I completely forgot about Google Docs, Sheets, Drive, Account Settings, Alerts and probably others all going Material too.
My remaining question is if it will now let me see which phone number or email address is labeled as "home" v. "work" v. "defunct" in the autocomplete area.
Chrome 40.0.2214.115 on Windows 7. I'll try on Linux when I get home.