checkpassword-pam is an implementation of checkpassword-compatible authentication program.
The checkpassword interface is described here: http://cr.yp.to/checkpwd.html
You could use the checkpassword-pam together with:
checkpassword-pam(8) manual page. README. NEWS.
Version 0.99 is available for download from the project page.
If you are running Debian Woody, consider using this unofficial apt-get repository: http://www.jarre-de-the.net/computing/debian/.
The checkpassword-pam project is being hosted on SourceForge.net.