If you have a huge blog, you might want to create a mailinglist for your readers as well. Now to start that mailinglist, you could get all the people who once commented on your blog, or just those who registered for your blog, into that mailinglist. If you're running on WordPress and want to do such a thing, you're in luck: my latest plugin allows easy extraction of those email addresses and other important parameters.
Check it out: the Users to CSV WordPress plugin.
good idea