I've cobbled up a very small checkpassword-ldap program. Good for authenticaing real system users - http://foobarco.net/checkpwd.html .
Also Felix von Leitner wrote an LDAP checkpassword for qmail pop3d and Courier-IMAP with his tinyldap client library. It's very small: http://www.fefe.de/tinyldap/ .
