If you often find yourself commenting on other people’s blogs (and there are many good reasons to do this), then check out Commentful:

Commentful is a service that watches comments/follow-ups on Blog posts, Digg submissions, Flickr pictures, and many other types of content.

After a couple days of using Commentful, I’ve it intuitive to use and a great help in keeping tabs of online discussions I’ve joined (the Firefox plug-in is a must – a small yellow circle glows at you whenever watched blogs have new comments. And, Commentful is free!

The major drawback is that watched posts automatically expire after 3 days. So, it is not currently useful for, say, watching comments here on Needmore Notes. I have a hunch that this limitation won’t last for long. Since Commentful is in beta, I wrote to inquire about adding paid subscriptions without expirations.

[UPDATE] Heard from the nice folks at Commentful. The 3 day limit is only for a short test period and limits will be soon (this is still free). It appears that there are no current plans for a paid version.