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.

Cirrus is a visualization tool that displays a word cloud relating to the frequency of words appearing in one or more documents. One can click on any word appearing in the cloud to obtain detailed information about its relativity.

This tool generates a word cloud of the top frequency words from a text document, with word size determined by its frequency. The user can specify how many words are to be included from the document, whether to apply a modified Glasgow Stop Words list, and the order in which the words are displayed. Clicking on a word in the resultant word cloud will generate a concordance for that word. This tool is currently only available for plain text documents. Should the user provide an HTML or XML text, all tags will be stripped and it will be processed as plain text.

This tool’s functionality has been incorporated into Voyant available online at www.voyant-tools.org.

TagCrowd is a tool for generating a frequency-based word cloud from a source text, with a free browser version available through the TagCrowd website. A commercial version may also be purchased, subject to a creative commons license.

SEASR's NGram Tag Cloud Viewer is a free tool for generating a tag cloud from a text hosted at a URL. It can also process PDF documents.

Although the page on SEASR's website is no longer active, it can still be viewed via the Internet Archive.

SEASR's Tag Cloud Viewer With Stemming is a free tool for creating a tag cloud from a text hosted at a URL, and can process PDF documents. While this tool is similar to SEASR's NGram Tag Cloud Viewer, it applies a stemmer to the text during processing.

Though the page for this tool is no longer available on the SEASR website, it can still be viewed via the Internet Archive.

TokenX is a free text visualization and analysis tool for XML documents. It offeres a web-based environment and can generate word clouds, highlight parts of text such as words, non-words and punctuation, KWIC (keyword in context) and more. TokenX also offers more playful options, such as word substitution, or an option to replace words with images.

DiscoverText is a cloud-based textual analytics tool available for purchase. It is designed to collect data from sources such as social media, blogs and a variety of document formats, and generate tag clouds and reports from the resulting corpus.

Trend Miner is a tool designed to enable portable open-source real-time methods for cross-lingual mining and summarizing of large-scale stream media. It combines elements from natural language processing, knowledge-based reasoning, machine learning, economics and political science. It is designed to access and interpret data from sources such as blogs, Facebook and Twitter.

Textal is a text analysis app for the iPhone and iPad offered by the UCL Centre for Digital Humanities. It creates word clouds from texts, websites, tweets and documents, then allows users to explore the statistics and relationships between words in the text. As a tool, it is intended as an introduction to to text analysis while expanding the functionality traditionally implemented in word clouds.