Simple Update Protocolwikipedia
Simple Update Protocol, or SUP, was a protocol developed by FriendFeed to simplify and speed up RSS and Atom.

FriendFeed

Simple Update Protocol, or SUP, was a protocol developed by FriendFeed to simplify and speed up RSS and Atom.
Employees of FriendFeed created the Simple Update Protocol to reduce the load put on sites by aggregators such as theirs.

RSS

RSSRSS feedsRSS feed
Simple Update Protocol, or SUP, was a protocol developed by FriendFeed to simplify and speed up RSS and Atom.

Atom (Web standard)

AtomAtom Publishing ProtocolAtom Syndication Format and Atom Publishing Protocol
Simple Update Protocol, or SUP, was a protocol developed by FriendFeed to simplify and speed up RSS and Atom.

Disqus

Disqus commenting widget
A few websites supported SUP for a while, meaning that updates from their services appeared in FriendFeed within seconds, until support was dropped, and eventually FriendFeed itself shutdown 2015-04-09. These sites include Disqus, Identi.ca, reddit.

Identi.ca

identi.ca
A few websites supported SUP for a while, meaning that updates from their services appeared in FriendFeed within seconds, until support was dropped, and eventually FriendFeed itself shutdown 2015-04-09. These sites include Disqus, Identi.ca, reddit.

Reddit

redditsubredditReddit AMA
A few websites supported SUP for a while, meaning that updates from their services appeared in FriendFeed within seconds, until support was dropped, and eventually FriendFeed itself shutdown 2015-04-09. These sites include Disqus, Identi.ca, reddit.

Brightkite

brightkite
Brightkite supported SUP but was itself shutdown in April 2012.

YouTube

YouTubeyoutube.comYouTube channel
YouTube supported SUP according to YouTube v2 API SUP documentation, yet as that reference says "The YouTube Data API (v2) has been officially deprecated as of March 4, 2014."

WebSub

YouTube Data API (v3) supports push notifications via PubSubHubbub instead and no longer has SUP section.