The LDAP server that I run is OpenLDAP (http://www.openldap.org). It's free and generally a good choice. Remind me to add links to U-Mich's LDAP server and Netscape's LDAP server someday, and maybe even stick a link u pto Novell's NDS crap... ;)

To get your clients to authenticate, they need to have an abstract translator for the LDAP lookups. padl software (http://www.padl.com) provides some nice utilities to do this, including a PAM module (http://www.padl.com/pam_ldap.html) and an NSS library (http://www.padl.com/nss_ldap.html). Most modern Linux systems, as well as Solaris and some? of the BSD's (and some others) use the NameService Switch and Pluggable Authentication Modules, so that makes the transition easy.

On the server, you'll want to have perl (http://www.perl.com) and the padl migration tools (http://www.padl.com/tools.html). That will allow you to just copy your existing system database over to the LDAP database so you don't have to retype a bunch of stuff.

In summary, you need
http://www.padl.com/download/MigrationTools.tgz
http://www.padl.com/download/nss_ldap.tgz
http://www.padl.com/download/pam_ldap.tgz
ftp://ftp.OpenLDAP.org/pub/OpenLDAP/openldap-stable/
and maybe my utilities?