QuickCal - Possibly my favorite application. Allows me add tasks and events to my calendar without ever opening iCal.
 - I rarely visit Twitter.com these days. With this app, there is just no need to. Control/Option/Command/T (^⌥⌘T) opens up a new tweet box. The menu bar icon lights up blue whenever I receive a notification. Tweets appear in the bottom-right corner via . It’s as simple as that.
Sparrow - My personal and school email are consolidated into one Gmail account, which I access via Sparrow. Like Twitter for Mac, the menu bar icon lights up blue whenever I receive a notification.
Dropbox - An easy way to sync files online between multiple computers.
Droplr - The easiest way to share files online. Just drag a file to the Droplr icon in the menu bar and it spits out a URL that you can share with people.
F.lux - Automatically adjusts your computer screen’s color after sunset for optimal viewing comfort. Great app. Highly recommended. 
 - This menu bar icon lights up blue whenever I receive a notification. Then and only then do I visit Facebook.com. ^⌥⌘space allows me to update my status right from my desktop.
(From . Also posted on .)