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.
An Open Source IRC chat applet that you can embed in your own website.
This is an applet version of the pure Java PDF viewer component offered by BFO. It uses a signed applet, so once trusted it can open files from your local machine and display them inside the applet, including support for multi page navigation, zooming, JavaScript and document signatures.
This is an electronic circuit simulator - you can modify and add components like switches, transformers and logic gates and see how they behave in the circuit.
JPC is an x86 PC emulator written as a Java applet. It's quite an impressive thing, having basically the same machine I had when I was growing up now running inside a web page. Plus, being able to play Prince of Persia doesn't hurt either...
An HTML editor applet, the JSyndrome applet is a full WYSIWG HTML editor that provides sophisticated editing and previewing of HTML documents and integrates with websites and Content Management Systems.
An applet from the University of St. Andrews that allows you to upload an image of someone's face and then perform one of many possible transformations to it, such as 'Manga Cartoon', 'Baby', 'Drunk' and 'Apeman', usually with weird and funny results.
VisualRoute is a graphical traceroute test to any IP address, which can help identify network performance or routing problems and identify IP address locations.
Well, despite the fact that we've had more than 3 kids in the house playing this against each other on several occasions, I didn't know that Runescape runs as a Java applet until Gary mentioned it last week.
Not much of a business case for this applet, but it helps to demonstrate the wide variety of uses for Java applets: this is the Super Mario Brothers game in an applet.
A page displaying a video of a Linux Magazine speaker about Apache 3.0, but the window displaying the video is a Java applet, which is surprising as Java's multimedia support has been less than impressive for a long time.
JScreenFix is a very useful Applet: it rapidly turns on and off random pixels to prevent monitor 'burn-in' and to fix 'stuck pixels' on LCD monitors. If you've had stuck pixels you'll know how annoying they can be - run this applet for 20 mins and you should get rid of them. How cool is that?
Project Scenegraph aims to provide scenegraph functionality at the Java level, and has a few cool JavaFX demo applets using the SceneGraph API.
Map 24 provide interactive street maps, complete with searching by address, points of interest and full directions, using an applet.
IRIS (Interactive Rich Imaging Software) was one of the most impressive demos at last year's JavaOne conference. It mixes applets with AJAX to provide a photo viewer and editing tool that connects to Flickr to allow you to see slideshows, edit photos and upload photos to your Flickr account.
just2easy provides a full WYSYWYG word processor in an applet, providing the usual text formatting and layout, as well as automatically giving each document you create a unique URL so it can be viewed by anyone with a browser.
This is a really impressive applet based website that allows you to fly around New York, like Google Earth, but also allows you to see details about businesses and buildings, post reviews of places and read others' reviews. I like the ability to click on individual buildings - very cool.