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.
