If you're using Mac OS X 10.4.* then you can try this with every Cocoa applications such as Safari, Pages, Keynote, OmniGraffle, TextEdit or even iTunes! Mind you, if you right-click (control-click / ctrl-click) over a word, you can check its definition using Oxford Dictionary but this new method will show that word's definition / thesaurus without having to launch Oxford Dictionary application.
Here's how it works:-
Go over any word on any Cocoa application, say Safari. Click "
command + ctrl + D" while you're over that word.
A small pop-up window will show up containing the definition of that word, you can also choose its thesaurus using the small drop-down list under the definition. To keep getting definitions of every word you hover above just keep holding down "
command + ctrl" after you release the "D" and move over other words with your mouse/trackpad.
Here's a video of that tip in action: [link]
Pros: Very easy, very helpful and looks unique.
Cons: Doesn't work on non-Cocoa applications (FireFox, Opera, Terminal..etc.)
* Officially posted on macosxhints.com