Systray.exe can display the following icons in the notification area:

Battery Meter                               PC Card Status                               Volume Control                               Quickres                               Task Scheduler
Other programs can also add icons:
Windows Explorer adds a printer status icon.                              Exchange or Outlook adds a mail status icon.                               The Accessibility status indicator program (Accstat.exe) displays keyboard accessibility icons.                              Microsoft Fax adds the Fax Rendering and Fax Status icons.

NOTE: See How do I hide System Tray (Notification Area) icons in Windows XP?