If you have a new plugin that you want to release to the Cytoscape community, you have a few options to advertise it.
Check that your plugin conforms to the requirements listed in the plugin tutorial (manifest and plugin.props files are required).
Go to http://chianti.ucsd.edu/cyto_web/plugins/pluginsubmittype.php and follow the instructions to submit your plugin.
Send an announcement to the announcement mailing list, which has hundreds of people on it. See How_to_send_an_announcement
Notes:
If you wish to host your own plugin download site you do not need to submit the plugin to Cytoscape. You will need to generate an xml file that the Cytoscape Plugin Manager can read, please see the download site tutorial for how to create this file. Please make sure to send the link to this xml file in your announcement email so that other uses can access it.