Tiger's Dictionary Lookup feature has been called the "use-it-anywhere" dictionary. Sadly, that's not true unless every application on your Mac is a Cocoa application that uses NSTextView and WebView, which are the requirements for accessing the dictionary via contextual menu or keyboard shortcut. That's not the case on my Mac. I spend a lot of my time in Carbon apps, most notably BBEdit. To use the lookup feature in BBEdit, I have to make a trip to the Services menu. That's pretty inconvenient for someone who likes to keep their hands on the keyboard. Services can be extremely useful, but if there are no keyboard shortcuts to accompany the desired Service, odds are good that I won't be using it.
Thanks to John Gruber, I can now use the built-in Dictionary Lookup feature in BBEdit and a handful of other applications I spend a lot of time in without missing a beat. John's recent article reminded me that I can assign a custom keyboard shortcut to the lookup service (and many other things) using the Keyboard Shortcuts tab in the Keyboard and Mouse preference pane. For some reason, I always forget that preference exists, so I'm happy to be reminded of it. The only problem with this method is that it requires you to select the word you want to lookup, rather than just hovering over it. I can live with that. If you can't, check out the Applescript in John's article, which uses GUI scripting in order to accomplish the same goal, without having to highlight the word.