I’ve made a lot of wordpress plugins. A few plugins have come from using wordpress as an internal intranet / groupware tool. I’m one of the top wordpress plugin developers. My wordpress profile is found here and a list of the plugins below:

1) Facebook Comments - Import your comments from Facebook to Wordpress
2) Allow Categories - Permission private / hidden categories by user
3) Private Files - Make sure users are logged in before they can view images / attachments (useful with allow categories)
4) YouTube Feed - Embed you tube videos in posts and detect Facebook feed reader, and convert to a link.
5) 3rd Party Authentication - Authenticate wordpress against gmail / google apps or email.
6) Post ToDo - Add a ToDo list to a post or page.
7) Limegreen - A plugin to read information from a limegreen update file.
8) WP Bible Gateway - Automatically insert links to bible gateway in your posts.

I also intend to eventually write a document mangement and SVN browser for wordpress. Oh, also I found this great site for plugin developers which documents the various “hooks” that you can use in plugins: http://wphooks.flatearth.org