The /etc/shells file is a list of valid shells on the system. Example from an Ubuntu 12.10 Desktop system:
# /etc/shells: valid login shells
/bin/sh
/bin/dash
/bin/bash
/bin/rbash
/usr/bin/screen
Example from a CentOS 6 minimal server:
/bin/sh
/bin/bash
/sbin/nologin
/bin/dash