The Generic ping plugin can be used to add a generic RPC ping service to your blog.
The Wacko is a simple renderer plugin that provides Wacko style formatting. This allows to easily create different markup/formatting, even on blogs that do not allow HTML.
You do not need this plugin with b2evo 2.0.x which has embedded support for categories.
Check Blog settings > Category List Widget
This plugin allows you to add CAPTCHAs to your site in the form of simple questions requiring simple text answers.
For example the site would ask "What animal is Bugs Bunny?" and the user would need to type in "rabbit" or "a rabbit".