In OS X 10.7 (Lion), Apple removed the 'Save As' menu item and replaced it with 'Duplicate'. Though similar, Duplicate does not function the same as Save As, and lots of people were unhappy with the change. Apple relented in 10.8 (Mountain Lion) by returning "Save As..." but they hid it as an optional menu item which would only be shown when you held down the Option key while looking at the "File" menu. (There is also a keyboard shortcut for "Save As..." in 10.8, but it is not very convenient: Command + Shift + Option + S.) This option was so well-hidden that most did not know it existed at all.
Here are two ways to return the “Save As” functionality in OS X. Both function by changing the shortcut for the existing function. One is via the Terminal, and the other through System Preferences.
First, quit all your apps except Finder and Terminal. Then copy and paste this command (as one line) into the Terminal window, and press Return:
defaults write -globalDomain NSUserKeyEquivalents -dict-add 'Save As...' ‘@$S’
That’s it! Launch any text-based application to verify “Save As” in the File menu. It should be noted that there won’t be a confirmation given in the Terminal window. If you’re unsure about using Terminal, perhaps the next option will suit you better.
If you’re less inclined to use Terminal, we can use System Preferences instead. From Applications, LaunchPad, or the Finder Apple menu Launch the System Preferences application, then open the "Keyboard" preference pane.
In the top menu of the dialogue box, click "Shortcuts" (labeled '1' below). Then in the list on the left side, click "Application Shortcuts" (labeled '2' below). Next, click the "+" button (labeled '3' below). Finally (as seen in the second image) enter "Save As..." in the "Menu Title:" field, and then press the keyboard shortcut that you want to use. In the example below I pressed Command+Shift+S.
Addendum: "Keep changes in original document": The "Save As..." command in early versions of 10.8 had an unexpected and likely unwanted side effect in Mountain Lion: it would save the changes in the new document (created by "Save As...") but would also save the changes to the original document.
However, Apple realized that users might not want that behavior, so in Mac OS X 10.8.2 they added an option "Keep changes in original document" which you can see here:
Option A: If you want to save the changes you've made in the document and then save the document with a different name, then make sure that the box is checked.
Option B: If you want your original document to stay as it was when you last saved it and create a new document based on the modified content of that document, then make sure that box is not checked.