While starting up your computer in X, hold down ð-V (Command-V). Instead of the regular mac startup screen you will view the extremely complicated process through words and commands. It's Unix heaven!
Can't log in to OS X? Try booting in "single user" mode by holding Apple-S on bootup. It will drop you directly into a shell. You might be able to fix whatever problems you're having from the CLI. Got me out of trouble once...
