WordPress Plugins can extend WordPress to do almost anything you can imagine. WordPress provides a free home for any plugin at the WordPress Plugin Directory.
If you have created your wordpress plugin, before promoting it would be better to check and test it with WordPress Plugin Repository and Subversion (also known as SVN) is software that allows you to easily store and manage your plugin projects.
Before promoting free check out the entire WordPress plugin repository on the following websites:

1. Request repository access

2. Updating WordPress with SVN

3. SVN help and tool downloads

4. Easy PHP

Download, install and code! Setup at a glance a pre-configured Windows, Apache, MySQL, PHP environment on your system or on a USB drive and develop or present locally your projects.

5. PHPSVN Client

PHP SVN Client class that can perform read operations from a SVN server (over WebDAV).

After finishing above steps and when the documentation is ready, it’s time to Promote Your New Plugin.

1. Add at WordPress Plugins Directory

2. WordPress Plugins and Hacks forums

Create New topic on this Forum and in Tittle must mention Your Plugin name. Use a simple but informative template to describe your plugin release.Also Don’t forget to mention:
1. Plugin Name
2. Plugin URL
3. Description

3. Weblog Tools Collection forum

4. WP Plugins Database

Good place where you must ad your plugin.

5. WordPress Plugin Compatibility List

Add your plugin to the WordPress Plugin Compatibility list. For the most up-to-date plugin compatibility data, please visit individual plugin pages on the WordPress Plugin Directory and look for the Compatibility widget
You need to register before you can add changes to the list.

6. Off course Never forget to Announce the plugin on your blog.

More info on these links:

1. Releasing and Promoting Your WordPress Plugin

2. HOW TO: 14 Tips to Promote Your Remarkable WordPress Plugin