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.

PhiloGL is an open source WebGL Framework for advanced data visualization, creative coding and game development. It includes a module system encompassing Program and Shader management, IO, XHR, JSONP, Web Worker management, Effects and Tweening, among others.

The JavaScript InfoVis Toolkit is a free set of tools for creating interactive data visualizations compatible with web standards. Visualization options include bar graphs, radial graphs, tree graphs and more, many integrating animations.

Bookworm is a free tool from the Harvard Cultural Observatory designed to generate data from searches on the Open Library and Internet Archives' collections of public domain books. It allows users to search keywords, including in multiple languages simultaneously, and narrow terms by country, gender, subject and header. At present, Bookworm is restricted to searching books published between 1820 and 1920 due to availabliity. The tool produces a graph showing instances of each keyword over time, enabling easy comparison of results.

Gephi is a free, open source interactive visualization and data exploration tool. Users can manipulate the display to uncover new facets of the data, enabling intutive exploration.

Juxta is a free, open source tool for comparing and collating texts, originally intended for comparing multiple versions of the same text. It offers several views and visualization options, including histograms and side by side comparison. Juxta is available as both a desktop application and as a web service that can be integrated into the user's own website.

WEAVE (Web-based Analysis and Visualization Environment) is a web-based visualization platform offered by the Institute for Visualization and Perception Research at the University of Massachusetts Lowell. It is designed to visualize any data for any purpose, and to accommodate novice and advanced users alike. It includes the ability to integrate, disseminate and visualize data at nested levels of geography. WEAVE is currently in beta. Both a web-based demo and a download version are available.

TXM is a free, open source text corpus analysis environment. Its features include concordance, collocate search, frequencies based on the CQP full text search engine and statistical functions based on R packages. It can export results in CSV, XML or SVG format, and runs as a standalone desktop application in Windows, OS X and Linux environments, or as a web portal (download the leaflet for more).

Quadrigram is an environment for data gathering, query and visualization, based on a visual programming language and intended for users with no experience programming or creating visualizations. At present, it contains 50 different visualizers.