novu

Knock

Knock is a customer engagement and notifications platform that gives developers a single API and workflow builder for sending messages across email, SMS, push, chat, and in-app channels. Instead of building and maintaining custom notification pipelines, teams define reusable workflows with preferences, batching, and delivery logic, while non-technical teammates can edit content and audiences without redeploying code. Knock pulls in real-time data from data warehouses, CDPs, and webhooks to drive dynamic audience segmentation and personalized content, and it offers AI-assisted workflow creation from natural language prompts. It's used by companies like Vercel, Webflow, and Medium to handle hundreds of millions of messages per month, with SDKs for 8+ languages and compliance support for HIPAA, SOC2, GDPR, and CCPA.

Courier

Courier is a multichannel notification platform that lets developers send email, push, SMS, in-app, and chat messages through a single API and journey builder, rather than integrating separately with each channel provider. It positions itself as "the messaging platform for humans and AI agents," adding an AI Agent node and AI-powered translations to its journeys and broadcast tooling. Note: the product's original domain trycourier.com now permanently redirects to courier.com. The platform includes an inbox component for in-app notifications, user preference management, and observability integrations, with pricing based on message volume rather than seats or channel count. The free Developer tier includes 10,000 sends per month with journeys, broadcasts, an MCP server, a CLI, and SDKs; paid usage is billed per send, and Enterprise adds dedicated support, RBAC, SLAs, and EU data residency.

Customer.io

Customer.io is a marketing automation platform that lets businesses send personalized messages based on user behavior. It supports email, SMS, and push notifications, enabling real‑time engagement across channels. The service provides a visual workflow builder, robust segmentation, and detailed analytics, helping marketers optimize conversion and retention without writing code.