This processing based applet visualizes the HTML DOM structure of any website, with different coloured nodes representing different HTML tags.
The Preservation of Favoured Traces is a Processing based applet that visually displays the differences between various revisions of Darwin's Origin of Species.
The Falling Sand game has been around for a while, and allows you to create paths, structures and flows from 4 falling sand elements. Simple, but oddly addictive.
An applet that visualizes current global emotions by scanning blog entries and tweets for occurrences of "I feel..." and "I am feeling...", then displays then in a dynamic cloud that can be filtered by a variety of criteria.
Bitlet is a browser-based video player for BitTorrent files - so you can stream torrent videos and watch them in your browser, rather than waiting for the whole file to download.
ThinkFree is an MS Office compatible applet-based office suite, allowing you to create and open documents and spreadsheets without installing any software.
The Twitter Venn applet allows you to enter 2 or 3 search terms and searches Twitter for matches, then presents the results as a venn diagram, which illustrates the rate of tweets containing the search terms in the various combinations.
This applet allows you to play the piano and will accompany you on different predefined songs.
This applet shows 3D orbit visualization of planets and small bodies (asteroids). This is quite interesting in itself, but especially today, as Asteroid 2009 BD is passing relatively close to the Earth today.
Cannon Defender is a classic "shoot stuff out of the sky" arcade game, implemented very nicely as a Java applet.
Left 4k Dead is an applet game written for a Java 4k coding challenge - it is inspired by the Left 4 Dead arcade game, but is implemented as a Java applet, in just 4k, which is a hell of an achievement. It's actually good fun to play.
The Thinking Machine is a Processing based applet that will give you a good game of chess, but the cool thing is that while it is thinking about its next move it shows you the potential moves that it is considering, using coloured lines.
This is an applet that allows you to design chemical structures in a WYSIWYG editor. The editor supports undo/redo, importing and exporting of diagrams and it can be controlled by JavaScript. I know nothing about chemistry, but the diagrams look convincing to me :-)
This site has a collection of pen and paper puzzles, such as anagrams, Sudoku, and spot the difference (pictured), all implemented as Java applets.
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.
This applet simulates the operation of the Enigma machine, used by Nazi Germany in WWII for encrypting communications. Source code is also available.
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 -:-)
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.
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.
An online jigsaw puzzle game, which lets you solve jigsaws of between 6 and 240+ pieces.
