Linux, Unix and OS/X determining what shell is running

When running a terminal window it is possible to determine the current shell by running the following from the command-line:
echo $SHELL

An example of invoking this from an OS/X terminal window is as follows:

As echo $SHELL demonstrates above, the current shell is BASH.

The title bar of the OS/X terminal actually show the same information revealed by echo $SHELL, namely bash: