Jason came to me a few years ago looking for a way to track the popularity and correctness of comments on his site. He knew he wanted to use the native WordPress comment system and that it had to be quick and easy for a visitor to make a judgment. That’s where I came in.
I built a small plugin that allows visitors to his site to “recommend” comments by clicking a link. The recommendation happens asynchronously, so the visitor never sees the page refresh. In addition, the visitor is prevented from recommending a single comment multiple times by a simple cookie tracking system. Finally, the site owner (in this case, Jason) can manually modify the number of recommendations that any single comment has (thus enabling them to correct for overzealous supporters or scripting attacks).
The best thing about this plugin, and one of the things that makes me so proud, is that the comment recommendations plugin is still in use to this day. That means the plugin has survived 5 major version upgrades of WordPress and has maintained compatibility both from a user interface perspective and a functionality perspective.