HTML painting
D3 is a beautiful, intuitive and incredibly flexible Javascript library. You can get started very quickly and also decipher more complex code very easily thanks to how it has been developed and presented. It is by far my favorite web tool and has taught me the little I know about Javascript. It makes you feel like an HTML painter. There is also a good free book I would recommend.
Hats off to Mike Bostock who has also made the home page and his personal examples all available and interactive via bl.ocks.org. It makes for a very user-friendly learning experience with a lot of good introductory and advanced examples. He also kindly hosts any D3 blocks you push up to a GitHub Gist in the same user-friendly format.