As a Mac switcher, my Mac's built-in spelling and grammar checking has been a huge productivity boon for me. I'm someone who often gets stuck on a word, and since nothing's ever good enough for me,
I've often wished that Leopard also included a built-in thesaurus. While that's not currently in the cards, there is an alternative. How many ways are there to say "Whoops?" Of course, Leopard does include a built-in thesaurus via Dictionary.app. See the continuation of this post for a screenshot. Thanks to everyone in the comments for keeping me honest.
If the Apple offering isn't to your liking, Nisus Thesaurus, a free app from Nisus Software, works as a standalone application and a system service. This means that it installs in the Services sub-menu of your Mac and is accessible from any program you use that is able to interact with the Services sub-menu. These applications include Mail, TextEdit, Safari, MacJournal, and countless others.
Once installed, using your new thesaurus is as simple as highlighting a word that you would like to look up, and pressing the Nisus Thesaurus Services sub-menu hotkey (Command -<). This will pop up the Nisus Thesaurus window with your word options only a click away. Select the word you would like to use as a replacement, press Command->, and voila; your new choice has replaced the original word.