Flock
RSS aggregator written in Java.
http://flock.sourceforge.net/

Hotsheet
A Java based desktop newsreader. Works on Windows, Macintosh, Linux and anything else with a Java Virtual Machine.
http://www.johnmunsch.com/projects/HotSheet/

JERSS
Allows seamless integration of RSS newsfeeds into websites. It is a Java servlet generating Javascript objects the web author can manipulate.
http://www.jayeckles.com/news/

NewsMonster
Mozilla and Java-based aggregator with RSS 2.0 support.
http://www.newsmonster.org/

nntp//rss
Enables RSS feeds to be read with NNTP newsreaders. Requires the Java Runtime Environment.
http://www.methodize.org/nntprss/

RSS Viewer
A Java based, open source RSS reader.
http://rssview.sourceforge.net/

Thinfeeder
Free, cross platform, multi-language RSS / Atom / RDF reader. Anti-aliasing, skins and multi-language support. Developed in Java, works under Windows, Linux, MacOSX and others.
http://thinfeeder.sourceforge.net/

BlogBridge
Windows, Mac and Linux based RSS feed reader, which is designed to allow users to easily manage and follow hundreds of feeds.
http://www.blogbridge.com/

RSSOwl
Uses the SWT Eclipse library to provide a cross-platform desktop application to read RSS subscriptions. Free software.
http://www.rssowl.org/

Feed'n Read
Free Java based open source newsfeed reader. Supports all current formats (RSS 0.90, 0.91, 0.92, 0.93, 0.94, 1.00, 2.00, Atom 0.30 and 1.00).
http://fnr.sourceforge.net/

TopFeed reader
A free reader that supports RSS, RDF and Atom based syndication formats.
http://home.iprimus.com.au/gautamsonu/topfeed/

AgileRss
A free desktop aggregator that supports all versions of RSS and Atom.
http://www.agilerss.com/

JRSST: A Java RSS and Atom Ticker/Reader
A simple, lightweight, RSS and Atom ticker/reader.
http://jrsst.sourceforge.net/