I want to be able to have a checkbox where a visitor can subscribe to comments via email when there is a new comment on the post in 1.8

I have [url=http://forums.b2evolution.net/search.php?sid=46b4f559aa394d7622f818c31d901788]searched for this feature[/url], but not found anything.

[url=http://www.village-idiot.org/archives/2006/07/02/wordpress-last-fm-cloud/]Here[/url] is an example of what I am talking about.