English Thesaurus is a very complete offline thesaurus. It is a comprehensive English thesaurus by Word Magic Software that can be accessed in its entirety without an active Internet connection. In other words, you can look up any word without being connected to the Internet. However, other extra features like pronunciation and spell-check require a connection.
Word Magic Software claim that English Thesaurus has 1,049,000 synonyms and 283,000 antonyms, which is a lot. The application only weights 160 megabytes, you can easily take it with you in a USB drive if portability is what you are after.
The graphical user interface is plain, but functional. There are two panes. The one on the left contains a search field and a list of words sorted alphabetically. The pane on the right displays the results of your searches or the synonyms and antonyms of the words that you click on the left pane. Most of the time, results will be shown as you type. However, I noticed that in some cases, this didn't work, and you have to press the "Enter" key to get any results. This happened with the word "Self-aware", for example. Other than that little problem, the application performs as expected. The definitions are good and you can click on words on the results to look them up as well.
At only $1.99, English Thesaurus makes for a great purchase for anyone who has the need for an offline thesaurus.