RSS-Bridge

RSS-Bridge is a PHP project capable of generating RSS/Atom feeds for a multitude of sources. You self-host it on your own webserver or run it in stand-alone CLI mode.

Supports a long list of content sources (aka bridges).

• List of public RSS Bridge instances

RSSHub

RSSHub (over 17k stars on Github, wow!). It even has a browser extension and a mobile app. Docs.
Via reddit

"Kill the Newsletter!"

Kill the Newsletter!, by Leandro Facchinetti. Converts email newsletters into Atom feeds.
You can use the hosted service kindly offered by its creator, or self-host it yourself.
Via reddit.

FiveFilters

Web service that lets you create RSS feeds for any web sites that don't support it natively, among other useful content-related services. Offers a free tier, no account necessary. Paying customers also get access to the code and the ability to self-host the service (FiveFilters provide Puppet scripts).
FiveFilters maintains an open-source repo with site-specific content extraction rules that is widely used.

PolitePol

PolitePol is a web service where you can create RSS feed for any page on the internet using
interactive XPath selection. Create either XML or JSON feeds.
Service is not FOSS; pricing appears to depend on number of feeds. Unless you sign up the service is quite hobbled (feed lifetime 14 days, and feed never updates).

Feed43

Web service that lets you create RSS feeds for any web sites that don't support it natively. Offers a free plan, with or without first registering an account, but the code behind the service is proprietary.

RSS Box

RSS Box by Stefan Sundin, supports:

• Twitter (but Nitter may be better, also see this excellent guide by FiveFilters)
• Youtube (YT actually offers RSS feed per channel, also consider Invidious)
• Vimeo
• Instagram (another option is Bibliogram)
• Periscope
• SoundCloud
• Mixcloud
• Twitch
• Speedrun
• Dailymotion
• Imgur
• SVT Play

Feed me up, Scotty!

Feed me up, Scotty!, by Vincent Tunru. Hat-tip to Marie Dubremetz.

Notifier

Notifier, by Kirill Maltsev.
Can generate RSS feeds from emails and Telegram channels.
Offers up to 100 notifications per month for free. For 4 €/month you could receive up to 20,000 notifications.
Via kickscondor.