OneSignal - Customer engagement platform used by over 1 million developers and marketers; the fastest and most reliable way to send mobile and web push notifications, in-app messages, emails, and SMS.
Pusher Beams - Send push notifications programmatically at scale
PushBots - Light SDK for mobile push notifications
Mentioned By - Learn when your site is mentioned by other sites for free
iZooto - Engage your mobile and desktop web users with intelligent web push notifications.
Now Push Notify - iOS does not support browser notifications today, but Now Push Notify makes this possible.