The OS X system menu's support for menu extras is a convenient way to access system and application settings and services. While a default set of menu extras such as the time and date, volume, wifi indicator, Time Machine status, and Spotlight menus are enabled, others can be turned on in their respective system preferences or application settings. Usually these menu extras stay to the right and out of the way of application and system menus, but in some instances they may run into the menus of various applications and be hidden.
Menu extras can usually be moved around by pressing the Command key and dragging them to a different order; however, this is not the case with all menu extras. Some such as the Spotlight menu are set to be stuck in place, but the ability to move a menu extra also depends on how the menu extra is coded since there are basically two ways to implement them.
The first type of menu extra is a more modern implementation that has the system treat it like a standard menu bar item, and these can be dragged around. All built-in menu extras for system preferences and Apple's services are coded this way. The second implementation is the older method that treats the menu extra more like a separate application; these are loaded along with an application or background tasks and will appear after the other menu bar items have loaded. These cannot be moved around, and are also placed to the left of all the other menu extras so they can more easily be occluded by the system and application menus.
Ultimately, your options for managing menu extras will depend on the types of menu extras you have; however, you can do some things to minimize the amount of space used by various menu extras and thereby increase the room for them on your system:
Change screen resolution: By default the system will output the desktop at the display's native resolution, but if you have decreased the system's output resolution, then the image on screen of the desktop will be smaller, and therefore menu items will be relatively larger and take up more space. Setting the resolution to a higher value in the Displays system preferences (preferably as high as possible for optimal image quality) will increase the available space in the menu bar.
Remove unused menu extras: While menu extras are convenient ways to access system services and settings in OS X, if you rarely use them, then you can remove them. For instance, I have the iChat menu extra enabled even though I rarely use iChat, so I can take it off the menu bar without changing my workflow. To do this for most menu items (at least those that are coded as true menu extras), just press the Command key and then drag the menu extra off the menu bar. You can always re-enable menu extras in their respective system preferences or applications.
Minimize menu extras: If you can, go to the settings for a menu extra and reduce its footprint in the menu bar. For instance, on laptops you can change the battery status indicator to show only the icon or only the text, instead of both the icon and text. Likewise, you can change the date/clock menu extra to show an analog clock or even to show different aspects of the time when kept in digital mode (AM/PM indicator, day of the week, or even the whole date altogether).