One of the plugins I ran across recently is markItUp!. markItUp! was created by Jay Salvat as a "universal markup editor". Depending on the settings you open it with, markItUp! can be used to edit with Html, Textile, Wiki Syntax, Markdown, BBcode, or even your own text markup language.
This site uses Habari as it's engine. Now Habari has several nice wysiwyg editor plugins, including TinyMCE, jWysiwyg, and NicEdit. However, I'd rather be able to see what markup is being put in the source when I write, so I've been using Habari's built-in editor, which is just a text area you type in. Any tags, you have to enter yourself.
But I'm lazy. Or, if you want to put it politely, I will optimize my productivity when possible. While I don't want want wysiwyg, I'm more than willing to let software do the work of putting in tags for me. When I found markItUp! it seemed the perfect candidate to become a markup plugin for Habari. Using the example of the jWysiwyg plugin, I was able to do so in fairly quick order. After contacting the author and Elijah at NumberBox, who had also worked on a markItUp! plugin for Habari, markUp is now in Habari's -extras repository for the community of Habari users.
Download markUp. Test drive it. Suggest improvements, or better yet, add them. :)