Butler
Who wouldn't like to have their own butler?
How about a butler for you on your computer?
Well, get your very own Butler __today__!
And, yup, it's Mac only.
It is an amazing little helper, and has worked nicely into my daily workflow. There are so many features that I still haven't discovered them all of them yet. I currently use the shortcut keys for entering abbreviations for launching applications, for doing a Google or Yahoo search, controlling iTunes, for launching the Unix shell, Terminal, and also the shortcut for the stacked pasteboard, which saves a user-defined number of past cmd-c commands. Almost all of the features can be assigned a user-defined keyboard shortcut, and all the searches are quite fast.
It isn't a Spotlight replacement, for it doesn't really index what's in documents, but instead is an app launcher and helper in general.
Oh, and, it's free (as in speech). I strongly recommend all Mac users try out this application, and donate some money to the developer if you like it and use it a lot.
It is this kind of software that makes the Macintosh Experience� so enjoyable for me.