I've been doing a lot of writing lately. I usually start a writing project in Word or NeoOffice, so that I can use an outliner to quickly organize and rearrange my thoughts. I also rely on the thesaurus in those programs.
But sometimes for various reasons I do my writing in InDesign. But InDesign doesn't have a Thesaurus, and I miss that. But thanks to Mac OS X "Services", I can access a thesaurus from within InDesign. Just select a word, and choose InDesign > Services > Look Up in Dictionary. The Apple Dictionary application will be launched, and the selected word will be looked up in the dictionary. From there, I can click on Thesaurus to explore alternate words, or Wikipedia to learn more about the word.
Other services that I've sometimes used in InDesign or InCopy include Show Address in Google Maps, Search With Google and Open URL.
These options in the Services section of theInDesign menu (aka the Application menu) are provided by OS X, and work in many Macintosh applications (but not Illustrator, Photoshop or Flash unfortunately). So this is a Macintosh-only feature. I don't know of a similar technology available on Windows.
In OS X 10.6, you may need to choose InDesign > Services > Services Preferences to turn on some of the services mentioned above. Only services that have a check mark next to them will show up in your Services menu.
I've also installed WordService and CalcService from DEVONtechnologies. These free downloads add a bunch of text and number processing features to the Services menu. Particularly handy is the ability to quickly insert the current date and time into your text, and the ability to do math in InDesign.
If you frequently need to shorten URLs in your text, check out this solution for Bit.ly, This is really slick. You just select a long URL in InDesign, choose InDesign > Services > Shorten URL with Bit.ly, and Bit.ly is contacted, the URL is shortened, and your long URL is replaced with the short URL. Click here for solutions for TinyURL and various other URL shorteners.
Warning, I encountered one serious bug while messing around with this. If I have a cell or any other portion of a table selected, and I choose InDesign > Services, InDesign crashes immediately. I'm running InDesign CS4 6.0.4 on Mac OS X 10.6.3.