On early UNIX, (when the Bourne shell was the most advanced Bourne-compatible shell), it lived as /bin/sh.
Now there are at least three Bourne-compatible shells in common use:

• ash - Almquist SHell; this was originally a clone of an some Bourne shell (sysvr4?), and deliberately lacks extended features. (There's several variants; we have busybox ash by default.)
• bash - Bourne Again SHell; GNU clone of bash with a truckload of features.
• ksh - Korn SHell; This is another classic UNIX shell, designed with a focus on scripting, as well as enhanced interactive use from csh, and backwards compatibility to sh. (There's several variants here, too...)
• zsh; less commonly used, but attempts to incorporate the benefits of bash, ksh, and tcsh. Most notable use is probably Darwin and derivatives.

On most modern UNIXen, there's no binary at /bin/sh; it's a symlink to some Bourne-compatible shell (usually with extensions). But it's always there, and should always be nominally compatible. So by writing scripts using a portable subset of what your /bin/sh knows, and using /bin/sh as an interpreter, your script can work on any UNIX.

Sven Mascheck has some more info on portability, and on shell history in general, at his site. Good reading.

But bottom line is: shebang for /bin/sh, and if it runs on ours (busybox ash), it's a decent bet it'll work everywhere else.