Portable OpenBSD ksh(1).
Because other operating systems deserve a good shell to use. Originally a port of ksh to FreeBSD. The default FreeBSD shell is tcsh. No one likes tcsh. DragonFly BSD should also just work. Later NetBSD support was added. NetBSD also has a ksh variant so it might be less useful there. Linux support, tested on Slackware and Ubuntu, exists. Darwin (Mac OS X) supported as well. Other operating system support welcome and appreciated.
Most files are public domain (see LEGAL). The rest are BSD or ISC licensed.