I am looking for a user management script for my site, which will allow my site visitors to signup using a form (by supplying username & password and a few other details), so that they can view the main contents of my site, by logging in. Also, the script should have facility for verification through email when the user sign up. Also, it should allow the user to change their password later, if they want to. Can anyone recommend me such a script (free or paid) which can work with CGI, PHP/MySQL or any other ?

I'd suggest having a look at Hotscripts (http://www.hotscripts.com/)
There are plenty to choose from, though it will depend on what you are looking for exactly.
If you wan to setup pay-per-view type service, then IBill (http://www.ibill.com) and others, might be more suitable.

What exactly are you trying to use it for??

Do you want something like: www.modernbill.com? Or do you mean something entirely other?

Because I don't really understand for what purposes you are wanting to use it.

No, not that advanced. Its for my personal site and I want to restrict the main contents of my site to visitors who signup using the form. So whenever they want to enter the site, they have to login with thier username and password.

I don't think there is anything like that. So I gues you should develop it your own or let it develop by a programmer. If you chose the last option then I would suggest that you let it program in PHP&MySQL. Because both programs are great :)

Have you looked on Hotscripts (http://www.hotscripts.com) ?
There are quite a few scripts that will do this kind of thing, or at least provide you with the codebase to develop something more complete.