Java Applets
Some applets require later versions of Java (it's not 1998 anymore...) If you're running Windows with Internet Explorer, you'll likely need to download Sun's Java Runtime Environment (JRE). (Not sure where to click on that page? Google can highlight it for you). Here is why Microsoft no longer supports Java.
Assuming you've installed the plug-in and are running the latest version of your browser, these applets have been tested and work on:
A port of Minesweeper to Java - with a facelift and new features. It puts other versions of Minesweeper to shame.
A paper and pencil puzzle in which you must deduce the locations of the hidden ships.
Another paper and pencil puzzle in which you must connect the dots following specific rules.
A port of the popular Minesweeper game to Java. This (lighter) version should run in older browsers as well.
This is an applet that I find quite useful. It takes a vocabulary list and quizzes the user. There are many quizzes already included for classes I've taken (BIOL 101 / 102, GER 110) or you can enter in your own. Worth checking out, especially if you are like me and hate writing (and losing) flash cards.
These applets convert infix notation to postfix notation and calculate the result. There's one for basic arithmetic and another to generate truth tables for boolean expressions.
This applet implements a monoalphabetic substitution cipher to encrypt a message. Type in a message, choose a cipher alphabet, and then view the enciphered message.
My final project for Math 121: Introduction to Computing when I was a freshman. If you're interested in how a really simple compiler works, then check it out.