I want to let visitors sign up for an email account (I have a lot of space) on my site. Does anyone know a free Perl, PHP or Java script I can run on cPanel for this? I was also looking for something for subdomains too but that's not for a while..

Try www.hotscripts.com mate,

here are some php email scripts, you will need to search thru them to find what you are looking for tho.
http://www.hotscripts.com/PHP/Scripts_and_Programs/Email_Systems/Web-based_Email/index.html

Are you looking for e-mail hosting on one domain, to let them use their own domain, etc..?

which software do you for customers?

if you need i can write you the scripts you need for free i am gaining experience :)))))