Webhook.site - Instantly generate a free, unique URL and email address to test, inspect, and automate (with a visual workflow editor and scripts) incoming HTTP requests and emails.

Umbrel - Run your personal server on a Raspberry Pi or Ubuntu/Debian, self-host open source apps like Nextcloud, Matrix, and Bitcoin node, and take full control of your data. For free.

Svix - The enterprise ready webhooks service, open-source and in the cloud.

Bitcoin Core - Bitcoin

Webhooks by Zapier - Move Your Data Instantly With Webhooks

websocketd - Turn any program that uses STDIN/STDOUT into a WebSocket server. Like inetd, but for WebSockets.