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.
Yahoo Pipes was what internet should have been. We're so many decades into computing and that kind of inter-tool linking has only barely been matched by unix pipes.
Many companies are working very hard to make that impossible unfortunately. For example you can't get posts from public Facebook groups automatically, although that would be a really good source candidate. They used to allow it, but... not anymore.