The program egowatch.pl is a Perl script intended to alert the reader about followups in newsgroups. It is best used in conjunction with Gnus and the functions found in ego-watch.el.
Sample use: Once in a while, I have a question for comp.os.linux.hardware. I don't often read that group, since I'm mostly interested in answers to my own questions, not to others'. After I post a question, I try to check a newsgroup for a few days to see what answers turn up, but it's easy to forget. It's also easy to miss answers that come surprisingly long after the original question.
This program takes care of that. When I post a question, I select the posted article in Gnus and hit F5. This writes some data to the file .newswatch/newswatch.in, consisting of newsgroup name, header and match information, as well as an expiration date. This is a text file, so you could write this data in the file by hand, but I doubt you want to.
A few times a day, this program runs as a cron job, checking for matches for each of the entries. For example, if I post a question, then I can set the program to check for every post with the same subject (as a substring) as my original question. If one or more matches are found, the program sends an email to me, with a short synopsis of the match. That way, no missed answers to my questions.
You can also ask for matches on the references header instead of the subject header. That way, if a thread is very long and most of it is dull, the program will only alert you to posts in which your message ID appears in the References field -- that is, posts which are directly down-thread of yours.
If you're in Usenet stalking mode, you could ask the program to look for posts with a given "from" header. Any header at all will do, and the matching is done via regexp, allowing for quite sophisticated tweaking.
To use the functions, you'll want the Perl source and probably also the emacs-lisp functions for Gnus.