It is very easy to create your own plugin, with a little bit knowledge of PHP, MySQL and HTML/CSS it is fairly easy to create your own plugin!
Let's have a look into the very simple plugin Open external URL in new Window. This plugin is included in the download package.
Let's start with the folder structure:
Your plugin should always contain a index.html (blank) file, so people can't browse through your folder using a web browser. The install.php and uninstall.php file needs to be in the package as well and should not be renamed. Check the files and all the available hooks and you should be able to program your own plugin in no time.