Mailgun has just released an awesome collection of open source transaction html email templates for common email notification like Action emails e.g. activate your account, reset your password, Email alerts e.g. reaching a limit, there was a problem, Billing emails e.g. monthly receipts and invoices.
These html based transaction email templates are highly customize, responsive and fully tested on versatile email clients apps i.e. Gmail, Apple Mail, Outlook etc.
The open source project is hosted on Github for easily download and contribute.