Adium - Minimalist Adium notifier
Sparrow - Mail.app client replacement, minimal with Gmail Shortcuts option. Plus with Yahoo support.
GeekTool 3 - RainMeter counterpart for Mac. Here’s what my desktop .
Divvy - Window Management with KeyBindings. You can get the free version.
Pomodoro - Time boxing tool. The best thing about this one is that you can see the logs, very important especially if you’re billing on a task/hour basis.
And the usual Mac menu bar items: Time Machine, DialUp (using HSDPA Modem), WiFi Indicator, Volume Control, Battery, Digital Clock at 24-hour format, and Spotlight.
(From Julius Francisco)