There are various ways to get into this situation...
If you've removed Safari from your Dock, the open Finder and navigate to the Applications folder and drag the Safari icon from the Applications folder back to the dock
If you've deleted Safari from the Applications folder and have not emptied your trash, open the trash in Finder and drag Safari back to the Applications folder, and then a second drag, from Applications to the Dock, if that's necessary.
If you've deleted Safari and have emptied the trash, use Time Machine or whatever you're using for your backups, and restore the file from there.
If you've deleted Safari and emptied the trash and have no backups, then you can download and install the 10.6.8 combo, and that should reload Safari, and refresh other updated pieces.
The roll-your-own approach (which is more advanced, and an approach that I don't generally recommend) is to download Pacifist and to fetch the file from a local copy of the OS X installer.
If you don't have some form of backups configured and operating, I'd encourage getting any necessary additional storage hardware, and getting Time Machine set up at your earliest convenience.