In a recent mail from Apple eNews:
3. Tips for Ten: Easy Application Switching
Want a fast way to switch from one open application to another?
Mac OS X version 10.3 "Panther" makes it as easy as typing Command-Tab.
Go ahead, try it.
When you hold down the Command and type Tab, Panther presents you
with a colorful dialog that displays icons for each of your open
applications. At the left side of this dialog, you'll see the icon
for your current application. Next to it (and highlighted), you'll
find the icon for the application you last used. Each time you
press Tab (without releasing the Command key), you highlight another
of your open applications.
And not only has it been standard in Windows since Win3.1, but you've been able to do this on MacOS since, ooh, about OS 8? Though to be fair I don't remember if you got "colorful icons" in those days.