D3.js is a library for working with visualizations in javascript.
Last year I wrote a new feature called Relationships for our in-house issue tracker to visually display dependencies (among other things) between issues. I decided to use D3.js because it provides flexibility, great layout engines, and doesn't abstract the DOM so we can use new features as they become available without waiting for D3 to update. Without going into too much detail, here are some D3 slides that better explain the library. I highly recommend the force layout.