I am planning to offer Linux shells access for my clients so they can use it for IRC and other kind of stuff. I've been noticed many provider offer such kind of access. I was wondering if there is a script or software program that can create log-ins?
I'd never recommend you to go with shell access. Practically it is not so worthy and if you offered such kind of services to your customers you will have to conflict with hard sysadmin and security experience to back it up.
WHMCS, cPanel, Direct-Admin, Plesk can execute this, however I would inform you against shell hosting accounts as they are an activity, which will establish some serious issues like misuse of authority so better way you should avoid to offer shell access to you clients.