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?