CommandBook adds general commands for Bukkit. It was made by sk89q for his server when he moved it over to Bukkit, seeing as though there weren't really any other general command plugins.
You need Bukkit.jar and WorldEdit.jar in a "lib" folder.
We happily accept contributions. The best way to do this is to fork CommandBook on GitHub, add your changes, and then submit a pull request. We'll look at it, make comments, and merge it into CommandBook if everything works out.
Your submissions have to be licensed under the GNU General Public License v3.