The Almquist Shell, ash for short. The incarnation ported to LIBC is a relative current NetBSD version. The vendor branch of ash can be found at http://svn.netlabs.org/libc/browser/vendor/NetBSD-ash/ and the current port at http://svn.netlabs.org/libc/browser/trunk/ash/.

For a bit more info see the ​Almquest shell in Wikipedia.