jsPlumb provides a way to "plumb" elements of a UI together.
This page contains examples of the various types of connections you can make with jsPlumb. There are several other demonstrations that offer a more focused look at some specific part of jsPlumb's functionality - pick one from the drop down on the left or cycle through Next/Previous.
jsPlumb requires one of jQuery, MooTools or YUI3. See the documentation for a more detailed discussion of requirements.
This demonstration uses jsPlumb 1.3.5, jQuery 1.7.1 and jQuery UI 1.8.13.