F1 - Brings up a Help window in many applications, including browsers, Microsoft Office and others
F2 - Renames a selected object
F3 - Opens search box in browsers
F4 - Displays the Address bar list in My Computer or Windows Explorer (Windows XP)
F5 - Updates the active Window. Useful to refresh a displayed page in a browser
F6 - Cycles through the screen elements in a window
F7 - Turn on/off Caret Mode (Firefox). Performs Spell and Grammar check (MS Word)
F8 - Extend selection (MS Office)
F9 - Updates fields (MS Office)
F10 - Activates menu bar options
F11 - Toggles between full screen and normal display in browsers
F12 - Opens 'Save as' dialog (MS Office)