Problem: Single Sign On for linux

Hi gurus,
I'd like to know your opions about Single Sign On (SSO) for linux (Debian). In my company, clients want to access to different services (FTP, HTTP, Mail, Web Applications ). I think about OpenLDAP and Proxy (Squid, Vulture) to resolve this problem but i'm not sure if they can. Are there others solutions for this SSO problem ? The more secure, the better it is.
Any suggestion will be welcome and thankful.
Kind regards,
Thanh Dat