ToneMatrix is a browser based sinewave synthesizer. I've seen some of these as DIY projects that you can build. This is the first I've seen I can play from my computer. Essentially you click on one of the little squares and it will play a tone. Click on a column and it will play a chord. It sequences from right to left.
From the descriptions of these syntesizers, I thought a device like this would have limited appeal. Once again I was wrong. It quickly ate up a hour of my time as I made up different sequences. It was extremely easy to build a pleasant melody in just a few minutes and then change it on the fly.
Fun. Entertaining. I've bookmarked this one for future moments of "messing around".
[Image captured from http://lab.andre-michelle.com/tonematrix]