Main specifications:
    Atom 1.0                https://tools.ietf.org/html/rfc4287
    RSS 2.0.11              http://www.rssboard.org/rss-specification
    RDF Site Summary 1.0    https://web.archive.org/web/20200130223216/http://web.resource.org/rss/1.0/spec
    RDF Site Summary 0.90   http://www.rssboard.org/rss-0-9-0
    RDF Site Summary 1.1    http://inamidst.com/rss1.1/
    JSON Feed v1            https://jsonfeed.org/version/1
    JSON Feed v1.1          https://jsonfeed.org/version/1.1
    h-feed                  http://microformats.org/wiki/h-feed

Common XML extensions:
    Dublin Core             https://web.archive.org/web/20200130223216/http://web.resource.org/rss/1.0/modules/dc/ (see also http://dublincore.org/documents/dces/ )
    Encoded content         https://web.archive.org/web/20200130223216/http://web.resource.org/rss/1.0/modules/content/ (updated syntax only)
    Syndication schedule    https://web.archive.org/web/20200130223216/http://web.resource.org/rss/1.0/modules/syndication/
    Google podcast          https://support.google.com/googleplay/podcasts/answer/6260341
    Apple podcast           https://help.apple.com/itc/podcasts_connect/#/itcb54353390 and https://help.apple.com/itc/podcasts_connect/#/itc2b3780e76
    Media RSS               http://www.rssboard.org/media-rss

Other XML extensions:
    RDF RSS enclosures      https://foz.home.xs4all.nl/mod_enclosure.html
    Other RDF RSS modules   https://web.archive.org/web/20071201195657/http://web.resource.org/rss/1.0/modules/proposed.html
    Raw Voice               https://web.archive.org/web/20170331023439/http://www.rawvoice.com/services/tools-and-resources/rawvoice-rss-2-0-module-xmlns-namespace-rss2/
    Comment API             https://web.archive.org/web/20081004031716/http://wellformedweb.org/story/9
    Comment Feed            http://sellsbrothers.com/12548
    Atom threads            https://web.archive.org/web/20081115230920/http://www.snellspace.com/wp/?p=194
    RSS 1.1 XHTML payloads  http://inamidst.com/rss1.1/payload

JSON Feed extensions:
    Cast                    http://help.tryca.st/cast-jsonfeed-support/#cast-json-feed-extension

Related reading:
    https://web.archive.org/web/20100111072733/http://rsscloud.org:80/walkthrough.html
    https://web.archive.org/web/20110927015220/http://diveintomark.org/archives/2004/02/04/incompatible-rss