SystemUIServer
Crowd-sourced documentation and tutorials by the Cocoa developer community
SystemUIServer runs the System Menu Bar.
Or at least the part on the right that handles NSMenuExtras and NSStatusItems (including the Spotlight menu). The part on the left is the application menu bar.
Other (not so important to Cocoa) duties that it takes care of include launching applications when CDs and DVDs are inserted, and putting up the dialog that says "You've inserted a blank CD, what would you like to do with it?"
If it crashes or is killed, it will automatically relaunch.
