Per-post email notifications

Just wondering if you have recommendations if I wanted to allow per-post email notifications. What I’m trying to accomplish is something like Wikipedia’s Watchlist, where users could sign up to receive notifications voluntarily. In our case, we would need support for custom post types and pages as well. This seems a bit different that what your plugin does, but do you have any thoughts?

http://wordpress.org/extend/plugins/peters-collaboration-e-mails/