TextMate: I do most of my writing for the Internet, and I use TextMate to do it. Microsoft Word is too bloated, has too many features, and uses a proprietary document format that changes every couple of years. TextMate is fast and easy and uses plain text, which lasts forever. 39 -- about $56. This is the most expensive tool on this list, everything else is $40 or under.
BusyCal. The vendor's marketing slogan is, "Think of it as iCal Pro," and that's a good description. It looks very similar to iCal, and works the same, but does a few tricks that iCal doesn't: It syncs to workgroups, makes it easier to enter and view event details without having to call up a pop-up window, has more sophisticated to-do management and alarm controls, and more. It's only a little bit better than iCal, but a little help is all iCal needs. $40 per computer.
EagleFiler for organizing documents on the desktop. A little bit easier to use and more powerful than the Mac's built-in Finder and Spotlight search. Also, unfortunately, a lot uglier. Still, I love it anyway. $40.
LaunchBar is a launcher utility for starting applications and opening documents using only the keyboard. LaunchBar also has a clipboard history; it remembers items you save to your clipboard, which is extremely useful. Some people prefer the free, open source QuickSilver, but LaunchBar is faster to come out with upgrades and offers better documentation. $35.
1Password password manager and automatic form-filler for Mac OS X. $39.95.
Textexpander typing shortcut utility. You program it so that when you type a short word, TextExpander expands it out to an arbitrary section of text, which can include rich text, graphics and images. I use "mmw" for my name, "ssig" for my e-mail signature, and so forth. $29.95.
Cinch easily resizes windows so you can put two of them side-by-side or one on top of the other, with each occupying half the screen. Surprisingly useful, $7. Read more at the Computerworld review: Cinch makes two-up, full-screen windows easy