jsplumb

This jQuery plugin provides a means for a developer to visually connect elements on their web page, in much the same way you might have seen on Yahoo Pipes. It uses Canvas in modern browsers, and Google's ExplorerCanvas script for stone-age browsers. Full transparent support for jQuery dragging is included, the API is super simple, and the compressed version of the script is just 11.6K.

You can see a demonstration here.
TweetPlumb is a great Twitter visualisation built using jsPlumb.

http://code.google.com/p/jsplumb/