While I'm at it, I thought I might add in an exciting library that's still in development - a force-feedback graph layout widget written in javascript that works across browsers. The guy making it has hooked it up to wordnet, which gives for some interesting demo data. At work, we have something similar to this, although ours can display text along the lines and allows you to select lines as well by hovering over them... so in a way ours is more advanced, but at the same time, it's not out in the wild like this guys.

And as one final thing - the most useful development tool you can get your hands on for javascript development work is called Firebug. It gives you an interactive command shell for doing Workspace like development - it also gives you an inspector that breaks up nodes in to styles, dom, layout, events, etc. It also gives you a debugger that links directly in to the source when things go wrong and finally it also gives you an ajax http messages monitor. Firebug is only available for Firefox.