allstarpuzzles.com

This site has a collection of pen and paper puzzles, such as anagrams, Sudoku, and spot the difference (pictured), all implemented as Java applets.

http://allstarpuzzles.com/

www.interactivepulp.com/pulpcore/

PulpCore is a really impressive framework for creating Java applet games that use smooth and rich 2D animations. It's open source too. Try the Milpa game demo, I had to double check that it was really using Java, not Flash.

http://code.google.com/p/pulpcore/

http://www.interactivepulp.com/pulpcore/

russells.freeshell.org/enigma/

This applet simulates the operation of the Enigma machine, used by Nazi Germany in WWII for encrypting communications. Source code is also available.

http://russells.freeshell.org/enigma/

www.nobleavatar.com

The Noble Avatar maker is an applet that allows you to create an avatar character and then export it to a PNG file for use in forums, games, IM applications etc. It apparently has over 15 trillion combinations of features, although I still couldn't get one to look much like me -:-)

http://www.nobleavatar.com/

javaboutique.internet.com/SpaceFlight/

The SpaceFlight applet simulates flying through space - you change direction by moving the mouse over the applet. It's a lot more impressive than that sounds, honestly.

http://javaboutique.internet.com/SpaceFlight/

mmcelhaney.blogspot.com/2008/10/iron-man-applet.html

This is a spinning cube, displaying a different image from the Iron Man movie on each side. Clicking on it stops it spinning, and moving the mouse around it changes the speed and direction of its rotation.

http://mmcelhaney.blogspot.com/2008/10/iron-man-applet.html

www.jigzone.com

An online jigsaw puzzle game, which lets you solve jigsaws of between 6 and 240+ pieces.

www.jigzone.com