YATCP allows you to have threaded comments (i.e. reply to existing comments) in your wordpress.
Using standard WordPress your comments are linear, despite the fact that WordPress provides the technical ability to thread comments. That way longer discussion Threads tend to get confusing, as the reader has to keep track who is responding to whom. YATCP (Yet another threaded comments plugin) tries to solve that problem by leveraging the possibilities provided by WordPress.
So YATCP came to live in a small private Project where I needed nested comments. Flexibility was a goal for this plugin so it should be easy to use YATCP in a way that fits your needs.
If you come across anything that is not solvable or obvious to see, don't hesitate to write me a Mail at firstname.lastname@example.org and we'll see what can be done. Finally YATCP is released under GPL, so feel free to get the Code and cook your own Plugin from that.