Novu

Novu is the notification infrastructure you'd build yourself if you had three months to spare. One API for email, SMS, push, in-app, and chat notifications — with a visual workflow editor, subscriber preferences, and digest/batching built in. It's the plumbing layer between your app and every notification channel.

If you're building a SaaS and need to send transactional notifications across multiple channels, Novu saves you from wiring up Sendgrid + Twilio + Firebase individually. Knock is the commercial alternative — cleaner, more polished, but paid. Courier is similar but more enterprise-focused. Building it yourself means maintaining templates, preferences, and delivery logic forever.

Best for SaaS founders who need multi-channel notifications without the infrastructure headache. The React component library for in-app notifications is particularly good.

The catch: the open source version requires self-hosting with MongoDB, Redis, and multiple workers — not trivial. The cloud version has a free tier but the pricing ramps quickly. And the project has pivoted its architecture multiple times, so some older tutorials and integrations are outdated.