PSFK, a news site with regular notes on visualization and infographics, has their own roundups like Data Visualization Tools, and refers to good resources and external roundups like Webdesigner Depot's exhaustive 50 Great Examples of Data Visualization.
Of course visualization is a big, if spare & silent, part of what Google is doing with Search Options, Hot Trends, etc. And while you've been able to visualize user activity live on Digg Labs for 2 years or so, they have expanded their offerings too. Mashable has covered some of this in roundups of visualization tools for social media: 16 Awesome Data Visualization Tools, Data Visualizations: 5 Beautiful Social Media Videos, and 6 Unique Twitter Visualizations.
Since some information is best conveyed in motion, Mashable also noted 7 Beautiful Data Visualizations (With Videos). Below is one example from Skyrails, a social network visualization system that can be used to visualize any data.
At one point the AE filter Useful Things from Profound Effects could help with data visualization, even with live data from the Internet (weather, etc.), but the technology was sold to SONY. Let's hope that Adobe's Office 2.0 Mashup isn't lost forever (unlikely given the momentum of Flash, Flex, and AIR) and that the AE team is interested in this area, even if it just means easy integration with Flash.
Update: Programmable Web News has categories for both visualization and mapping.
Update 2: SIGGRAPH 2009 is hosting an Information Aesthetics Showcase "in recognition of the increasingly prominent role that information visualization and data graphics are assuming in our digitally mediated culture."
Update 3: Keith Lang recommends Information Visualization, Second Edition: Perception for Design; you can read a mostly complete FREE sample of Information Visualization on Amazon.com.
Update 4: Check out InfoGraphic Designs: Overview, Examples and Best Practices.
Update 5: (July) see A pseudo-random collection of Information Visualization links from Generator X and Art Against Information: Case Studies in Data Practice on Fibre Culture, as well as blogs Ben Fry, Data is Nature, Jer Thorp's blprnt.com, Pete Search's Only Connect, and Robert Hodgin's Flight 404.
Update 6: (November) CNN noted the growth of visualization in A new way of looking at the world.