Mac OS X dictionary

At least once a week (and more likely once a day) I need to look up the spelling or meaning of a word. Interactive spell-checking is built into many Mac OS X applications (like TextMate), so that saves me when I'm typing. But a cool thing that's also built into many Mac OS X applications is the ability to easily get a dictionary definition for a word in a document.

For instance, you can do this when using Safari. If there's a word on a web page you don't know, just hover the mouse cursor over the word and press [Ctrl][Command][D]. After a few moments a small Dictionary window is displayed with the definition of the word you're hovering over. After reading the definition just click somewhere else in the document to discard the Dictionary window.

I believe this feature works in Mac OS X 10.4 and newer versions, and only in Cocoa applications. For instance, it works in the Safari web browser, but doesn't work in Firefox (which is not a Cocoa application).