Text/word clouds
A lot of dataviz experts don't think much of word clouds, considering them both unserious and unoriginal. You can think of them as the tiramisu of visualizations -- long-ago trendy, now overused. But some still enjoy these graphics that display each word from a text file once, with the size of the words varying depending on how often each one appears in the source.
IBM Word-Cloud Generator
What it does: Several tools mentioned previously can create word clouds, including Many Eyes and the Google Visualization API, as well as the website Wordle (which is a handy tool for making word clouds from websites instead of text files). But if you're looking for easy desktop software dedicated to the task, IBM's free Word-Cloud desktop application fits the bill.
What's cool: This is a quick and easy way to find frequency of words in text.
Drawbacks: Because it's trying to ignore words such as "a" and "the," the basic configuration can miss some important terms. In early tests, it didn't know the difference between "it" and "IT," and completely missed "AT&T."
Skill level: Advanced beginner. This app runs on the command line, so users should have the ability to find file paths and plug them into a sample command.
Runs on: Windows, macOS X and Linux running Java
Learn more: Check the examples that come with the download.
Social and other network analysis
These tools use a pre-Facebook/Twitter definition of "social network analysis" (SNA), referring to the discipline of finding connections between people based on various data sets. Investigative journalists have used such tools to, for example, find links between people who are involved in development projects or who are members of various boards of directors.
An understanding of statistical theories of network node analysis is necessary in order to use this category of software. Since I've only had a very basic introduction to that discipline, this is one category of tools I did not test hands-on. But if you're seeking software to do such analysis, one of these might meet your needs.
Gephi
What it does: Billed as a Photoshop for data, this open-source beta project is designed for visualizing statistical information, including relationships within networks of up to 50,000 nodes and half a million edges (connections or relationships) as well as network analyses of factors such as "betweenness," closeness and clustering coefficient.
Runs on: Windows, Linux, Mac OS X running Java 1.6
Learn more: Try this Quick Start tutorial.
NodeXL
What it does: This Excel plug-in displays network graphs from a given list of connections, helping you analyze and see patterns and relationships in the data.
NodeXL merges the older and current definitions of SNA. It's "optimized for analyzing online social media -- it includes built-in connections to query the APIs of Twitter, Flickr and YouTube, allowing you to draw networks of users and their activity," according to Peter Aldhous, San Francisco bureau chief for New Scientist magazine.
It also handles email and conventional network analysis files (including data created by the popular -- but not free -- analysis tool UCINET).
Runs on: Excel on Windows
Learn more: Download this free NodeXL tutorial (PDF) by science journalist Peter Aldhous.