Here's some for the OS X users out there(hey, a Mac is still a *nix box in this case).
Q: How do I open an application/file/folder from the terminal in OS X?
A: OS X includes a special command to do this. The "open" command will simulate a double click on any file/folder, allowing you to open them from the terminal. Just type "open <item's name>" to open it. Ex: to open the calculator, I could type "open /Applications/Calculator.app", which would open the Calculator no matter where I am in the file hiarcy. If I were already in the Applications folder, I could use "open Calculator.app". Please note that the vast majority of OS X applications are so-called "packages", which have .app on the end, and while not visable from the Finder directly, .app is needed with the "open" command. Also of note that when opening Applications with this command, they may not always open and generate errors instead. Running "open" as root usually solves this problem.
Q: How do I enable the root user in OS X?
A: The root user is "disabled" by default, and needs to be enabled if a user wants to gain full root access(although 99% of the tasks you need to do can be done through sudo, so enabling root isn't reccomended). To do this, you need to first open the NetInfo Manager in /Applications/Utilities. Once it's opened, go to the "domain" menu, then to "security - authenticate". From there, enter the username and password of a user with admin access. Once that's done, go to "domain - security" again, and click on "enable root user". After you do that, OS X will pop up a warning saying that root doesn't have a password, at which point you'll need to assign a password to root, and confirm it. After this, the root user will be enabled, and can be disabled in a similar mannor.