RSS

Really Simple Syndication, a fairly better-organized alternative to RDF, also XML-based.

Miniflux, an opiniated feed reader with Fever API support (writen in Go, uses Postgres as backend)
Page2RSS, a service to track web page changes.
RSSOwl, a Java-based aggregator.
Raggle, a Ruby-based CLI aggregator.
RSS Merge (uses LastRSS)
Planet, a flexible aggregator that powers a number of sites.
Hep - a very nice Twisted Python aggregator with a POP3/IMAP interface.
FeedBurner - slices and dices feeds (also converts between RSS and Atom). I expect it to start adding advertisements somewhere along the line.
rss2mail - Perl implementation
Mimir, RSS feeds via IM using Jabber
Tapestry - Comics via RSS
RSS Autodiscovery using PHP
nntp://rss (read RSS feeds with a News client. Neat.)
RSS Display (a Perl CGI)
Ultra-Liberal RSS Parser
XSLT blogroll for MovableType
Onyx RSS Parser - a PHP parser
RSS to IM Gateway
A Python Bayesian Classifier called Reverend that might well be useful for rating RSS feeds
Magpie, a PHP RSS parser
rss2pop3
Aggreg8
Atom and RSS Feed Library for PHP
Mailfeed - a PHP script that generates an RSS feed off a POP3 or IMAP mailbox.