Mac OS X Boot Up Commands
If you come from a Windows/Linux background, you will be used to boot loaders and boot keys for performing particular functions on bootup. Key's to access the BIOS, boot to Safe Mode and more.
While the Mac OS X boot process is streamlined and it hides the actual processes that are run, it does still include a number of special keys that you can use during the boot process to perform a range of useful functions.
These commands work in Leopard through to Mountain Lion. I'm sure they probably work in earlier versions and will likely work in later versions too.
Boot from a CD/DVD
Press and hold the C key as soon as the white boot screen appears or the startup sound plays to tell your Mac to boot from the optical drive.
Eject a Disc
Every once in a while a CD or DVD won't be formatted correctly or may be corrupted, preventing it from display in Finder, meaning it may not eject when pressing the eject key. If it won't, reboot the computer and as soon as the white boot screen appears or the startup sound plays, press and hold the eject key.
Force Boot to OS X
If for some reason your computer is not booting to OS X correctly, such as if you have just installed Windows through Boot Camp and the install gets stuck part way, you can force the computer to boot into OS X. Simply boot the computer and hold down the X key as soon as the white boot screen appears or boot sound plays.
Display Boot Menu
If you are running multiple operating systems you can choose which one to boot to (instead of the default configured one) by pressing and holding down the Option key as soon as the white boot screen appears or the boot sound plays. Once the boot menu appears, you can select which operating system to boot to.
Prevent Start-Up Applications
The closest thing to Windows Safe Mode is disabling start-up applications, so you are booting to just OS X with nothing else running. To do this, hold the Shift key when clicking the Login buttong. If the login screen doesn't display with your configuration, hold the Shift key while OS X boots in order to display it.