Blog/Feed Sites
Any WordPress-based website: {WEBSITE}/feed
Blogger: http://{USERNAME}.blogspot.com/rss.xml
Tumblr: https://{USERNAME}.tumblr.com/rss
Mastodon: {WEBSITE}/@{HANDLE}.rss
Content Sites
Flickr: https://www.flickr.com/photos_user.gne?path={USERNAME}&details=1, then look for the link at the bottom
Internet Archive: RSS Feeds and the Archive
Pinterest: https://www.pinterest.com/{USERNAME}/feed.rss
Pinterest — board: http://pinterest.com/{USERNAME}/{BOARD}/rss
Wikipedia: https://en.wikipedia.org/w/index.php?action=history&feed=atom&title={PAGE TITLE} or https://en.wikipedia.org/w/index.php?action=history&feed=rss&title={PAGE TITLE}
Chat/Forum Sites
Hacker News: Hacker News RSS
Reddit: Guide to RSS and Reddit
Reddit — subreddits: https://www.reddit.com/r/{SUBREDDIT NAME}.rss
Stack Overflow: https://stackoverflow.com/feeds/question/{NUMERICAL ID}
Stack Overflow — other: Hidden/non-obvious Stack Exchange RSS feeds
Telegram: https://tg.i-c-a.su/json/{CHANNEL NAME} or https://tg.i-c-a.su/json/{CHANNEL NAME}
Audio Sites
iTunes: https://itunes.apple.com/lookup?id={NUMERICAL ID}&entity=podcast
SoundCloud: https://feeds.soundcloud.com/users/soundcloud:users:{CHANNEL NUMERICAL ID}/sounds.rss
Video Sites
Twitch: Twitch stream RSS generator
YouTube: https://www.youtube.com/feeds/videos.xml?channel_id={CHANNEL HASH ID}
Others
You can often get private feeds by publishing to a feed (e.g., cloud storage updates, public notes), which can be very useful for communication on projects.
Most news outlets and independent podcasters have RSS feeds for their articles, and frequently with curated specializations for your convenience.
Evernote: https://www.evernote.com/pub/{USERNAME}/{NOTEBOOK}/feed
eBay — searches: add “&_rss=1” to the end of a search URL