Wordle is an online toy for generating word clouds using the text you provide.  Text can be submitted by providing an URL or by pasting raw text into an input.  The most frequent words from the text are then used as the source for the resulting visualization, where frequency is used to determine the size of each word.

You may customize the output by choosing from a variety of display options (layout, font, colours) or use the randomize feature to get a random visualization.  Additionally, there is built-in stop word support for many common languages, and the option to change the word case.

Links finds collocates for words and displays links between them using a force directed graph. It shows term frequencies in proximity to keyword. It is a visualization and shows a web of terms.

RapidMiner is an open source data mining tool with a GUI interface, available in both a free, unsupported 'community version' and a for-pay, fully supported 'enterprise edition'. It utilizes a standardized XML interchange format for processing, and is compatible with common relational databases and spreadsheet applications.

Circos is a free visualization tool for creating publication-quality infographics and illustrations. Its visualizations are arranged in a circle to facilitate exploring relationships between objects and positions, which also has the benefit of offering a high data-to-ink ratio. Though promoted for the sciences, it is highly flexible and can visualize datasets from a wide variety of fields.

TimeRime is a free, web-based timeline application for creating, viewing and comparing interactive timelines. This application requires registration, and timelines created within the application are public.

The Google Ngram Viewer is a free, web-based tool affiliated with Google Books. It displays a graph showing how a set of user-supplied phrases have occurred in a corpus of books over a selection of years. The tool includes functions for part-of-speech tagging, ngram compositions and word dependencies, to name a few.

TextGrid is a virtual research environment for text-based humanities scholarship. It offers a variety of tools and services for collaboratively creating, analyzing, editing and publishing texts. The TextGrid environment is split into two components, the Laboratory housing the tools and services, and the Repository for preserving reserach data and making it available to other scholars.

Tom Sawyer Perspectives is a commercial software package for data visualization and analysis. It provides a graphical software development kit and preview environment, including layout and Tom Sawyer Software's proprietary data visualization reference architecture. The SDK is particularly aimed at developers seeking to create data-oriented visualization applications, and permits users to customize with toolbars, context menus, and editing behaviour, to name a few. Applications are designed to enable design sharing and reuse, including via web application deployment. The Java version of Tom Sawyer Perspectives is available for Windows, OS X and Linux environments, and .NET for Windows only. Evaluation copies are available on request; no prices are quoted on the site.

Prefuse is a Java framework for creating interactive information visualization applications from the University of California at Berkeley's Visualization Lab. It can be used to design standalone applications, visual components within other applications and web applets.

RDF Gravity (RDF Graph Visualization Tool) is a free, open-source visualization tool from Salzburg Research's Knowledge Information Systems Group. It supports RDF graph structures and OWL ontologies, including multiple RDF files, and offers a variety of filters for its visualizations.