Books:

The Grammar of Graphics, Leland Wilkinson
Visualizing Data, William S. Cleveland
The Visual Display of Quantitative Information, Edward Tufte
Information Visualization: Perception for Design, Colin Ware
Show Me the Numbers: Designing Tables and Graphs to Enlighten, Stephen Few

Tools

Tableau (pros: one of the best data exploration tools, free for open data; cons: somewhat costly ~$1,700)
Pentaho Reporting
Pivot (Microsoft) http://www.getpivot.com
Many-Eyes    http://many-eyes.com
Verfiable        http://verifiable.com
TimeSearcher    http://www.cs.umd.edu/hcil/timesearcher
Parvis        http://home.subnet.at/flo/mv/parvis
Improvise        http://www.cs.ou.edu/~weaver/improvise/

GGobi        http://ggobi.org
Interactive, brushing, etc.

Tree Network Tools

GraphViz        http://www.graphviz.org
NodeXL        http://www.codeplex.com/NodeXL
GUESS        http://graphexploration.cond.org/
Pajek        http://pajek.imfm.si/doku.php
TreeMap         http://www.cs.umd.edu/hcil/treemap
Workbench    http://nwb.slis.indiana.edu/
 

Programming Tools

processing.org        A popular graphics language
protovis.org        Visualization tools for JavaScript
flare.prefuse.org    Visualization tools for Flash
prefuse.org        Visualization tools for Java
modestmaps.com    Mapping tools for Flash/JavaScript

People / Blogs

Andrew (bloger)
Nathan (bloger)
Jeffrey Heer (visualization librariries)
Katy Borner (visualization of science)
 

Reference

Some of the recommendations are by Jefferey Heer (an expert in the area) given at the MediaX 2009 workshop

Various

smoothScatter produces a smoothed color density representation of the scatterplot, obtained through a kernel density estimate.