Timo Weingärtner has graciously offered to take over the maintenance of pam_pwdfile. Its development will henceforth be hosted at https://github.com/tiwe-de/libpam-pwdfile.
PAM, or pluggable authentication modules, is a very effective method by which applications can abstract the way that they authenticate and authorize users. Using PAM for example, a whole system can be switched from normal /etc/passwd to shadow passwords without recompiling a single binary, as was the case with older *ix systems. Linux being my OS of choice, I have had most contact with Linux-PAM, as maintained by Andrew Morgan. Please see the Linux PAM pages at http://www.us.kernel.org/pub/linux/libs/pam/.
We required a Linux PAM module for authenticating using arbitrary password files containing username:crypted_password pairs, and this was not yet available for Linux PAM. As is the way with nicely open operating systems, I was able to put one together quickly, and you can get the source here. Since then, patches for extending functionality have also been contributed and integrated with pam_pwdfile. You can use this module to have different sets of passwords for each of the PAM-based services on your systems.
This module is available as part of Debian, where it is known as libpam-pwdfile. It also exists in the FreeBSD ports collection.
Warwick Duncan has written a very useful utility (command-line and CGI) to manage password files that can be used by pam_pwdfile. You can read all about chpwdfile and download it from the chpwdfile homepage! (Hmmm, it seems that the chpwdfile site has dropped off the edge of the internet. I’ve mirrored the source tarball of chpwdfile here. Remember that you can also use the apache htpasswd utility to manage your password files from the command line.)