There is an excellent WordPress plugin for allowing readers to subscribe to comments on a post. It is not my work: it was created by Mark Jaquith and Jennifer (a.k.a. ScriptyGoddess).
The only problem is that it requires you to install files into the root WordPress files, whereas plugins are recommended to be designed in such a way that all such files go into the Plugins directory.
Thus, this is my edited version that places all of the files properly in the Plugins directory. The current version is 2.0.8. (Requires WordPress 2 or higher).
I have contacted the creator and suggested he incorporate these fairly straightforward changes into the original plugin. Until that happens, this page is simply here for anybody else who wants this improvement.