www.thinkfree.com

ThinkFree is an MS Office compatible applet-based office suite, allowing you to create and open documents and spreadsheets without installing any software.

www.thinkfree.com

www.neoformix.com/Projects/TwitterVenn/view.php

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.

http://www.neoformix.com/Projects/TwitterVenn/view.php

koolpianoz.com

This applet allows you to play the piano and will accompany you on different predefined songs.

Needs Java 6 to run properly.

http://koolpianoz.com/

JPL Small Body browser

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.

http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=2009%20BD;orb=1

madsmx.tripod.com/cannon.html

Cannon Defender is a classic "shoot stuff out of the sky" arcade game, implemented very nicely as a Java applet.

http://madsmx.tripod.com/cannon.html

www.mojang.com/notch/j4k/l4kd/

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.

http://www.mojang.com/notch/j4k/l4kd/

turbulence.org/spotlight/thinking

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.

http://www.turbulence.org/spotlight/thinking/

http://www.processing.org/

Thanks to Rex Guo for the tip.