Here is my current and most immediate pick. The list will grow as I use them in real life.
Spotlight
I never really used Spotlight, even on my relatively fast MacBook. Instead I had Butler as an application launcher and the standard Finder for file searches (even though the Finder used Spotlight technology).
Now, Spotlight is all this and more.
- it displays applications first, so that you can really use it as an application launcher. Even though it is not as sophisticated as Butler, it does the job very properly.
- searches are faster than on Tiger.
- it calculates as you type a formula. I used PlainCalc for that before, and even though PlainCalc is much more powerful than Spotlight, the later is currently just what I need most of the time.
- it acts as a (English) dictionary by showing you the first line of the definition of the word you type, clicking on the result will bring you to the Dictionary application.
Spotlight on Apple's "New features" page.
Dictionary
Dictionary now has a set of 3 Japanese dictionaries (Shogakukan) and also accesses Wikipedia in the main available languages. It is faster than launching Safari to find a reference online. The only drawback, when compared to Safari, is that Dictionary does not display the language versions links of the article you are reading which does not allow the user to jump to the target language in which the reference is sought.
Dictionary on Apple's "New features" page.
Automator
Automator now has a "Record" function that allows it to remember mouse activity on any application's menus. That allows for quick "macro" recording in applications that are not well integrated with OSX and/or that do not support AppleScript. Java applications for translators come to mind !
Automator on Apple's New features" page.