I have just finished my first plugin. It is, as they say, rough as a cob. It allows you to display an image with the author's name based on the number of posts they have made. You can download it here:
http://www.brightandearlyblog.com/bbPress/downloads/display_rank_images.zip
You can see it in action at http://www.brightandearlyblog.com/bbPress and then looking at any topic.
Suggestions are greatly welcome. I have submitted it to the development center plugin repository.