I recently discovered that SmartOS includes a package for the zsh shell. I installed it with:

$ pkgin install zsh

Coming from a Linux background I attempted a chsh command, which failed since this command is no available on SmartOS/Solaris. After a little bit of digging, I found the solution on unix.com.

$ passwd -e

After you enter this command, you will be asked for your password followed by the location of the new shell. For me zsh was located in /opt/local/bin/zsh.