I am a big fan of HNNotify. I have had a link to hn notify in my profile for a while now. There is one glaring problem with the service: in addition to notifying you to comment replies it sends an email for every top level comment on stories you submit. I sent the owner an email a while ago pointing out the issue and it sounded like the system was set up in such a way the the developer could not differentiate between comment/submission replies.