Tools
D3.js (successor to Protovis). This is a Javascript library to do in-browser visualizations. The Github project contains source and examples. The examples page gives an indication of what can be done with D3.js. One particularly useful bit would be using treemaps to browse hierarchies of data. The biggest challenge here is getting data to the browser; it’s the web, so paradoxically getting access to remote data is easier than data on the local machine. So if I were using this to visualize data, I would need a server to provide the data, even if it was a local server.
Processing is a Java-based tool that can be used for visualization. One advantage to using Processing is that there are a lot of users, a lot of tutorials, and a lot of books. However, it is not a visualization toolkit per se.
Raphael is another Javascript library. It’s forte is as a mechanism to create SVG graphics from Javascript, so it is more akin to Processing in that you can build up visualizations with it.
InfoVis is a Javascript library for visualization. While it doesn’t seem as slick as D3.js, it has an impressive pedigree and some high-profile users (the Obama White House, for example). The author of InfoVis has two other related projects, PhiloGL (a Javascript WebGL framework), and V8-GL (Javascript bindings for creating 2D-3D hardware accelerated graphics).
R is a software environment for statistical computing and graphics. If you’re working with large amounts of data and you want to analyze or visualize it, R can handle it. It’s a language and a set of libraries which are being added to all the time.
Nodebox is a family of visualization tools.
Gephi is a cross-platform open-source visualization toolkit.
Other resources
visualizing.org is a website/community for people who do visualization.
visualizing data is a blog on visualization, with a monthly collection of “best visualizations”.
R Spatial Tips is a collection of tips for handling spatial data in R.
Tulp Interactive is the work of Jan Willem Tulp, an amazing data visualization expert.
visual complexity is a web site devoted to the visualization of complex networks. In particular, the blog is worth reading.
Graphic Sociology is a blog by Laura Norén where she analyzes visualizations in very long, thoughtful deconstruction articles.
information aesthetics is a blog that explores the relationship between creative design and visualization.
visual.ly is a showcase and marketplace for visualization.
vizualize is a blog about visualization.
Ben Fry’s blog (the author of Processing).
Eyeo Festival is perhaps the premier conference for visualization.