There is plenty of remarkable work being done over at Dofi-Blog in both the Java and Processing languages. Although the blog is written predominantly in Japanese, there are several compelling and engaging interactive games and toys that should not be missed regardless of your language of choice. Strongly recommended: World of Sand.
Liquid Webtoy is a Java-based application very similar to the Falling Sand games we're all so fond of. Rather than playing with solid elements, however, this game is focused on two things: liquid and color. Fill the screen with water, slosh it around with your mouse and add different shades to the mix. It can be as relaxing or chaotic as you like and is a great time-waster.
This simple and strangely compelling toy was just released onto Web featuring a primitive drawing interface with which to create a track for a little character to slide upon. Save and load tracks and even try to attain objectives to increase the enjoyment from this creative little sandbox webtoy.
Personal Universe is a puzzle game that lets you play with physics. Using sets of colored blocks you must build moving machines that help you complete tasks. Just as the name implies, Personal Universe gives you the materials to let you build a living universe all your own. It's filled with possibilities and offers a surprising amount of freedom to explore and experiment at your leisure.