Skitch: For capturing screen-shots.
QuickCal: I use less because I want to see my appointments and more because I was an early user of 30boxes on the web and loved the ability to write an english language appointment, e.g. Oil Change tomorrow at 9am.
TypeIt4Me:There are plenty of apps that do what TypeIt4Me does, but I’ve used this for a while and it never lets me down. I use it for two main purposes, shortcut typing and correcting my spelling on words that I often misspell. So if I have to type in my Gmail address, I created a shortcut and I just type m-y-g-m without the hyphens and it changes to my actual email address.
Dropbox: I think everyone has Dropbox at this point.
SizeUp: Easily size up your windows into half or quarter of a screen via keyboard shortcuts. Super handy.
Pastebot: I use this to paste something from my Mac that I want to use on my iPhone or iPad rather than type it out.
Spirited Away: Set which apps you want to fade away in the background after a certain period of not using them.
Pomodoro: I just started this a couple months ago and find it helpful. Work for 25 minutes, play for 5, repeat.
Hazel: Folder automated actions. I use this to monitor clean up my several folders including my downloads, desktop, and images.
Caffeine: I use this one constantly. Turns off all screensaver and screen shut-off actions.
LogMeIn: Log in to my Mac from my iPad.
: Turn my iPad into another display of my Mac.
: I attempt to send all my notifications through Growl so I can turn them all of at once when I wish and they all show up in the same spot when I leave them on.
MobileMe: I sync my contacts, calendar and assorted other mac pieces to mobile me, including my photos.
Time Machine: In addition to using Crashplan and Dropbox, I have at-home backup using Time Machine to a 500gb external hard drive.
Spaces: My on-again off-again love affair with Spaces is on again. I do most of my daily work in Space 1, leave Omnifocus by itself in Space 2, put all my music and chat in Space 3, and all my writing e.g. Scrivener, TextMate in Space 4.
Assorted Apple stuff: Bluetooth, Wi-fi, Volume, Battery, Time, Eject Drive, Accounts and Spotlight, which unlike many other people, works really well for me.
(From Kevin Farner)