Multi Email Clients

I want to expand the Email clients that I offer on my server.
I currently have Squirrelmail as my client, but have seen other ISP's that offer WebMail, Horde and Squirrelmail as the front end choices for customers.
This is a great idea...Any how-to's or tutorials on setting up a multi choice?
Also, anyone using ZPanel as a free control panel? anyone using a different one? {other that Webmin}

thanks,

Frog