Subscribe to Comments is one of the most famous and useful WordPress plugins of all time. When I first setup my blog, I was dumbfounded trying to figure out how to enable the checkbox "Notify me of updates to comments" like every other blog has.
It took me a long time before I realized everyone is using this ancient and unsupported plugin. I did my homework, took a deep breath, and installed it anyways. (The only old plugin on my site!)
To my surprise it has worked well so far and everything functions as it should. There is a bug in the Tools > Subscribe to Comments page where it shows a blank name (probably me, the site admin) with 11 subscriptions. But it's not the end of the world.
Wish someone would pick this up and continue development for current WordPress versions.