There are numerous free, customizable email templates available online, such as Email Blueprints from MailChimp, Ink from ZURB and Antwort. All of these templates are responsive and can be customized and edited in any HTML-ready email client. It is best to select templates based on what they will be used for. For example, MailChimp's templates may be better for newsletters while ZURB offers templates intended for showcasing products.
For simpler needs, several Chrome Extensions allow easy creation and customization of templates for Gmail. Newsletter Creator for Gmail by Flashissue offers a drag and drop editor for creating newsletters from common parts. It also helps users to manage subscriptions. Gorgias allows users to create emails from customizable templates as well, but is primarily focused on increasing productivity on everyday emails.