What are some alternatives?
When comparing Roost and OneSignal, you can also consider the following products
PushCrew - Push Notifications for Websites
Pusher Push Notifications API - Send push notifications programmatically at scale
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
iZooto - Engage your mobile and desktop web users with intelligent web push notifications.
PushOwl - Re-engage website visitors with push notifications
Urban Airship - Mobile marketing solutions across all major mobile platforms.