Without the user taking some type of action, you can't get a visitor's email address. They simply don't "carry" this information with them to the websites they visit.
However, there are a couple of websites that are using software that 'pops' up a window and says -- "do you want to subscribe to our newsletter"? If you click yes, a message is dropped in your outbox and send to the originator. The process has been around awhile, but it's a bit sneaky.
Ben Prater
BlueFoxLabs Web Development