I’ve been using the Salesforce Marketing Cloud (formerly ExactTarget) regularly for mass custom email sends. Marketing Cloud is a reliable CRM tool that provides reporting and analytics to give you feedback on what is happening to your emails after they are sent.
The nice team over at Zurb (the team that brought us the popular CSS framework - Foundation) released a cool responsive grid framework for emails that has been battle tested. This framework is Foundation for Emails.
As you know, one has to always be aware of the many operating systems, devices, and email clients that are being used these days. Testing is one thing, but having a solid framework to start with can be a huge help.
Pro tip: Don’t forget to use their inliner.
Also, here are a few that I’ve been working on for newsletters and other announcements. Feel free to clone and change for your own purposes.