Yahoo pipes. It was so great at creating rss feeds and custom workflows. There are replacements now like Zapier and n8n but loved that. Also google reader which is mentioned multiple times already.
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It has the advantage of being open source, has well defined and stable APIs and a solid backend. Plus 10+ years of constant development with many learnings around how to implement flow based programming visually.
I used the Node-RED frontend to create Browser-Red[^2] which is a Node-RED that solely executes in the browser, no server required. It does not support all Node-RED functionality but gives a good feel for using Node-RED and flow based programming.
The second project with which I am using Node-RED frontend is Erlang-Red[^3] which is Node-RED with an Erlang backend. Erlang is better suited to flow based programming than NodeJS, hence this attempt to demonstrate that!
Node-RED makes slightly different assumptions than Yahoo! Pipes - input ports being the biggest: all nodes in Node-RED have either zero or one input wires, nodes in Yahoo! Pipes had multiple input wires.
A good knowledge of jQuery is required but that makes it simpler to get into the frontend code - would be my argument ;) I am happy to answer questions related to Node-RED, email in bio.
[^1]: https://nodered.org
[^2]: https://cdn.flowhub.org