Dynamic Campaigns by Sigstr - Share the most relevant content in every email
Canned Emails - A minimal site with prewritten emails
PushMarket - Automate personalized emails for ecommerce (YC15)
analyze.email - Test the spam-y-ness of your emails
InboxSDK - First usable API to write Gmail Apps for 1B active users
TestSubject - Test how your email subject line displays on mobile devices.