D3 is so awesome. The other day a friend showed me Spotify Serendipity project, which used D3, and I was floored by the beauty of how visual data can show so much. A picture is truly worth more than a thousand words.
Here are some of the JS libraries that have made my JS development life much easier. I know that libraries change and new ones sprout, but currently here are a few that I have kept in my toolbox.