A number of applications and utilities are included with OS X to help new users get started with a variety of tasks in using their systems, including taking photographs with Photo Booth, chatting with iChat, using calendars, editing text documents, and referencing items in the Dictionary.
These tools and utilities are useful options in OS X; however, there are instances where one may inadvertently be deleted or otherwise end up missing from the Applicaitons or Utilities folders. In cases where these applications are rarely used, this might go unnoticed for a while, but can cause some confusion and delays when the user finds them missing. This is especially true for Utilities, which are useful to help in troubleshooting.
If an application included with OS X is missing from your system, there are several ways to restore it:
If you have Time Machine set up, then go to the Applications folder and invoke Time Machine. Browse back until you find the missing application and then click the "Restore" button to copy it back to your current system.
If you have another Mac that's running the same version of OS X as your current system, you can copy the application directly from that computer. The applications included in OS X are all the same version, so there should not be a problem as long as the versions of OS X match.
If you do not have a backup or another Mac available, you can use the package management utility Pacifist to extract the lost application either from the included OS X restore DVD or from a retail OS X installation DVD (whichever is the newer version).
To do this, download Pacifist and install it, and then insert your OS X DVD. From within the program, click the "Open Apple Install Discs" button to load the OS X installation DVD and you should see a list of the available packages that will be installed on the system in a default OS installation.
Browse through the packages until you locate the /Applications folder and find the missing application in it. With the application selected, click the "Install" button at the top of the Pacifist window, and proceed with the installation.
After the file is extracted and installed, be sure to download and run the latest Combo updater from Apple to ensure its packaged components are fully updated. Though installing from the Combo updater directly might be considered, many times Apple only includes the updated components of applications in these packages and not the whole application itself.
The last option is to use the OS X installation DVD to reinstall OS X. In the past, Apple used to include options in OS installers to select individual utilities or system components to install, but this feature has yet to make its way to OS X. The only Apple-supported option is to boot off the OS installation DVD and perform a standard install, which in Snow Leopard will perform an "Archive and Install" method of installation.
This will allow you to preserve your applications and user settings so you should not see a difference after the install, but the underlying OS components (including the Apple-supplied applications) will be refreshed. As with using Pacifist, after reinstalling be sure to fully update your system using the latest "Combo" updater for OS X, which will ensure the restored application is fully compatible with your system.
Keep in mind that most of Apple's included applications are self-contained so they can be installed manually using some of these methods; however, some other applications from Apple or third-party vendors may include numerous shared resources and require a full reinstallation to get functional again.