There are myriad of customization options in Mac OS X—some are in plain sight in System Preferences and elsewhere while others involve manual editing of plist (property list) files or tricking the system into thinking one file is actually another. One of the most powerful interface tweaks you can make is to the menu bar, and while you can enable menu bar extras through System Preferences and elsewhere, doing so can take some sleuthing.
Mac OS X is built for multiple users. As such, each user has a home folder with that user’s preferences and settings. But there are common resources shared by all users, and these resources are for the most part located in the System and Library folders at the root level of your hard drive. Before I go further, I should note the importance of taking care when modifying anything in either of these folders. One misstep can lead to a completely nonfunctional computer!
The menu bar extras I mentioned before are located at Macintosh HD/System/Library/CoreServices/Menu Extras. All you need to do is drag the files in this folder up to where you’d like them to live in the menu bar. Some of the menu bar extras will not “stick” up there because they are not supported on your system. For example, the ExpressCard menu item will not stick if used on a MacBook without an ExpressCard slot. If you use Mac OS X Server 10.5 or 10.6, you can realize power savings with the CPU menu bar item, which allows you to disable and enable processors or processor cores as more or less power is needed.
Another item in the CoreServices folder is the default desktop picture, which is shown at the login window. You can replace this file with another jpeg of equal resolution and name it DefaultDesktop.jpg to change the desktop picture at the login window.
Your mileage may vary on these hints. Whatever you do, make sure you’ve backed up your computer before trying anything.