Earlier quoted context omitted.
It's not just domain registrars, I reset the password of my basecamphq.com account (which stores very confidential project information) last week, and received this email: Hi -name-, Can't remember your password? Don't worry about it — it happens. We can help. Username: -username- Password: -password in plain text- Please keep your password safe to prevent unauthorized access. It blows my mind that even 37signals fal…
Not exactly a list, but: http://plaintextoffenders.com/
And another one to add to the list: my brother's small business uses British Telecom for email hosting. Their control panel stores the password in plaintext.