AirPort
To to enable or disable a wireless networking card or join and create wireless networks.

Battery
Only available on Apple portables and will display the amount of power in the battery.

Bluetooth
Connect to and manage wireless devices and the Bluetooth features of your Mac.  For example, the wireless Apple keyboards and mice.  It is also used to discover Bluetooth enabled cellphones 

Classic
Obtain status and and control Apple Menu items from Classic environments

Clock
Manage the OS X clock and change how it is displayed.  For example, you can opt to display an analog or digital clock

Displays
Used to adjust the screen resolution of monitors and laptop displays.  You can also enable or disable mirroring.

Eject
Enable the opening and closing of optical drives via a mouse click.  Please read the following.

Fax
View the status of the fax service in Mac OS X.  Easily monitor as you send or receive a fax.

iChat
Easily start a chat, show your current iChat status, view and manage Buddy lists