Popup the dictionary HUD.
Shift, just after logging in, to prevent Finder windows from opening and startup items from opening, for this boot.
Eject a stuck CD or DVD.
Start up in single user mode, which is a minimal BSD Unix environment. You can run /sbin/fsck -fy from the CLI, which will force-correct any disk problems. If you get a message that "FILE SYSTEM WAS MODIFIED" this means fsck found and repaired problems. Run it again until no error messages show: i.e.: ** The volume (name_of_volume) appears to be OK **. Exit single user mode by typing reboot.
Force a "Verbose" startup. Show Unix console messages during startup, and see what is going on behind the scenes as your Mac starts.
Display the Open Firmware screen.
Zap the Parameter RAM (PRAM). Start holding the keys prior to the grey screen after boot, hold them through the reset and an automatic reboot, through the second startup tone. Resets settings such as: display / video settings such as refresh rate, screen resolution, number of colors, startup volume, speaker volume, recent kernel panic info if any, DVD region setting. Be sure to go into System Preferences afterward and set the clock and timezone, and speaker volume.
Start from external drive (or CD).
Show the icons of all startup disks, for selection.