Thunder Mail - Cheap and simple transactional emails API with Amazon SES
MailClark - The Slack bot for external communications
Inbox Ignite - Prevent your emails from going into spam
Mailjet - All-in-one email service provider that provides maximum insight and deliverability results for both marketing and transactional emails.
PigeonBot - Send emails via Slack with the /email command
Silk - An Interactive web canvas for "Light Painting"