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.
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.
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.