Dimensional Fun
2D
Phun is out for OSX! Okay, it came out a few weeks ago, but I just got around to downloading it. Phun is a 2-D Physics sandbox.
At this time, I have no idea how I’ll use this in class, but I have a new prep to plan for next year and a summer that is mostly unplanned.
3D
Via information aesthetics I ran across Tag Galaxy, a 3-D1 representation of photos on flickr by tag. Each planet represents photos of related tags. Of course I had to see what the math tag looked like:

I had planned to post the screen shot of the “planet” of photos. However, the first photo I checked was not licensed under a Creative Commons License. So, be careful.2
4D
I’ve got nothing, sorry. Although I do keep bugging a friend to work on a time machine. Alas, he hasn’t come through yet.3
1Obviously it is really two dimensional. Also, I think it is more of a planetary system than a galaxy.
2In a strange coincidence, the first photo I checked for CC belonged to Courtney.
3I know, I know. He hasn’t built a tesseract either.