Ones with good demos appreciated, as they can be good for jump-starting data visualization projects with both vendors and clients who don't know the ballpark of what's possible.
What you need will depend on how exactly you wish to deploy visualizations. If you are interested in making your custom visualiztion, start with D3.
If you are into deploying visualizations that can synchronize with each other I have an ongoing project where I experiment with the idea. It is made for genetics & bioinformatics but you can use it for pretty much anything else.
Sebastian
I’d suggest D3.js. Regarding ideas and examples checkout these two ladies, they know their field.
sxywu.com
visualcinnamon.com