The Springz library allows you to attach two objects together (in 2D space) with a spring, and simulate how they pull/push each other.

Boring when used between two objects, it becomes interesting when you attach thousands of springs with different strenghts and distances between hundreds of nodes, and let the simulation determine the best configuration of those nodes. (I wrote this library specifically to create a social network diagram from a large amount of data. I had written it first in Javascript + SVG, and it was working, but too slowly: 3 hours to render the first frame, and 12+ hours for each frame after that.)

Despite the very visual nature of this application, this library doesn't know jack about graphics; it just knows how to push/pull theoretical objects around in theoretical 2D space. Making this visible to the user is up to you. (My application reads in XML data and then outputs a few frames of SVG pre-rendered animation.)

The documentation for the Springz class lists the key features at the top of it. You can read this laboriously-written documentation, and download the file itself, from: here (click on Springz.rb under the Files listing).

If you happen to have the Adobe SVG plugin (freely avail for Win/Mac/Linux/Solaris) installed, you can see the JS version of this library in action at -- click the green rectangle to start the simulation, and then drag objects around and/or click the orange button to scatter them about.