Pushover - Real-time notifications on your Android, iPhone, iPad, and Desktop
Amazon SNS - Fully managed pub/sub messaging for microservices, distributed systems, and serverless applications
Pushbullet - Pushbullet - Your devices working better together
push4site.com - Push4site.com service instant browser push notifications.
Gotify - a simple self-hosted server for sending and receiving messages
Join - Join is a marketplace for influencers and advertisers to collaborate.