I've created a new plugin for WordPress 2.0 and newer. It uses the WP usermeta system to create "subscribers" for your posts.
It's a simple install: just upload the file in your plugins directory and activate it.
Visitors to your blog can "subscribe" to receive emails of comments for a certain post, simply by checking a box in the comment form.
They can unsubscribe by clicking a link in the email they receive, or they can manage their subscriptions (both adding and deleting) by logging in under their profile. Users with sufficient permission (e.g. editors and administrators) can manage others' subscriptions. The subscription manager uses the nifty WP 2.0 Ajax-delete system to unsubscribe.
It's "beta," but it should work just fine.