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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Then all those sites I used to post on stopped supporting rss one by one and finally pipes was killed off.
For a while I used a python library called riko that did the same thing as pipes without the visual editor. I have to thank it for getting me off php and into python.