Dropbox: A really important service for my study, but also for my personal use. I love the possibility of sharing folders to colleagues or friends. A must have app for me…
Delibar: This is a menubar client to organize and use your Delicious or Pinboard bookmarks - in combination with the Safari extension it’s a killer-app! Love it!
Sparrow: A lightweight Gmail-client. It fits my needs perfectly.
: I think everyone knows it…
CoverSutra: Great iTunes controller which gives you the ability to search your entire music library over a Spotlight-like searchbar, displays album-artwork on your desktop, it also offers really nice iTunes player controls and more…
: Don’t  know why this app still has a place in my menubar… horrible weather-forecast, but at least it looks nice… ;)
: Gives you the possibility to bookmark webpages and open it via the menubar icon…. I abuse it as a “perhaps I will read it later” button…
: Altough it’s a bit expensive, I really love this little helper in my menubar - it detached Date-Line from my desktop. 
Transmit: FTP client of my choice. Great UI, great functionality!
Slim Battery Monitor: Nomen est omen.
DropIn: The best extension you can get for Dropbox, adds some really nice & useful gimmicks.
Screens: Mac Client for Screens to control your Mac with your iPhone / iPad.
Facebook: Hopefully Google+ will eliminate it from my menubar…
Radium: A menubar radio player that exactly does what it promises. Has a huge selection of radio stations included.
Time Machine 
Bluetooth
Volume
Analog Clock
User Switching
AirPort
: I use it in combination with strongVPN, at the moment mainly to use Spotify here. But there are some good other reasons to use it…
Spotlight
(From Joresch)