zsh is one of the unix shells that comes with OS X. I've written a few comments about why you might want to make zsh your default user shell. Briefly, it is most like ksh, the user-unfrienly but powerful shell prefered by serious shell programmers, except that it is in fact the most user-friendly and powerful interactive shell. In other words, it is simply better than everything else. zsh is to the other unix shells what OS X is to other operating systems. If you appreciate OS X, it is likely you will appreciate what zsh has to offer.