Services
Notifications
The users need to know! Email, SMS, Push, etc.

EmailJS
No server side code required to send email at all.

Firebase Cloud Messaging
Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably deliver messages at no cost.

Mailgun
Email sending service.

OneSignal
For sending Android, iOS, and Web push notifications.

PopcornNotify
Email and text messages with one JavaScript API.

Postmark
Transactional email sending service.

Sendgrid
Email sending service. Has an email builder tool as well.
Sendinblue
Email sending service. Has an email builder tool as well.

SparkPost
Email sending service with analytics.

Twilio
Twilio offers powerful APIs for text messaging, voice, and video.