Feedly

Feedly is an RSS reader, which makes it sound old-fashioned, but for a newsletter writer who needs to track dozens of sources — competing newsletters, news sites, blogs in your niche — it's still one of the most efficient ways to do it. Instead of checking twenty different sites, everything lands in one feed you can scan quickly.

The organizational features are what make it useful specifically for curation: you can group sources into boards by topic, star items to come back to, and build something close to a research pipeline for each issue, rather than relying on memory or scattered bookmarks to remember what you wanted to write about.