WordWeb is a handy utility that gives you the definition, synonyms, and antonyms of any given word without requiring you to go online, though it can connect to the Internet for expanded functionality should you want it.
The interface feels like an expanded version of any major word processing app's dictionary function, giving users an instant familiarity--even if they've never used WordWeb. After you type a term into the Lookup dialog box, the word's definition will appear in the interface's top pane, along with tabs that link to various wiki functions.
The bottom pane offers tabs for such items as synonyms, antonyms, and related words. The app's most notable feature is its hot key support; you simply use a hot key combo when the cursor is pointed over any word, and WordWeb will pop up onscreen with all pertinent information.
There are tools for changing the database to one of several English variations, altering the text size, and disabling WordWeb from recognizing offensive terms. The depth and functionality of WordWeb would be worth paying for, but thankfully you don't have to redefine your budget to get it--it's free.