See TweetFreq:

TweetFreq: Visualizing Twitter Update Behavior

TweetFreq is an application written in the Processing enviornment that visualizes the twitter update frequency of various collections of twitter users (for example, influential journalists, celebrities, CEOs, politicians, and news organizations

The collection of TweetFreq visualizations can be found in this Flickr Set:

http://www.flickr.com/photos/ajstarks/sets/72157618491647464...

More on TweetFreq Design, Implementation, Examples and Usage:

http://www.scribd.com/doc/22231126/TweetFreq-Visualizing-Twi...

There's also the Twitter Search Grid:

http://www.flickr.com/photos/ajstarks/sets/72157622763006024...