I know plenty of us come from a command-line background and like to see what's happening during the boot-up. By simply running this command inside Terminal on OS X, you get to see the boot messages (appropriately called "verbose mode") instead of a dull, blue screen while your Mac starts up.
sudo nvram boot=args="-v"
There are actually a few other interesting things you can do with the nvram command, but among them are some things that can cause your Mac to not boot. I do know that if you boot with the -s argument, it will go into single-user mode without getting prompted for the root password, but that's typical BSD stuff.
Posted by Ax0n