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.