No, there is no safe way to know the sites using your plugin.
Let's take a look at the options:
Add a marker to the rendered HTML, a comment or a meta tag.
This would still require a search engine to index these markers and tell you when they are found. But what's more important: your users would serve these extra bytes on each request to each user. What a waste of resources!
Phone home via email
I have seen plugins sending an email in an activation hook with the site address (and the admin email address). This is obviously unethical. Never collect any data without consent.
It is illegal too in countries with strong privacy laws like Germany. You could get sued in those countries, and due to formal cooperation between authorities a sentencing in Germany would reach you in other countries of the EU too.
Embed a tracker in your options page
De facto the same as above. Plus, you'd have to fiddle with SSL setups to avoid security warnings on the user interface.
In some setups (intranets for example) it isn't even possible to load external content; your plugin would fill the error log with very unpleasant messages.
So what could you do?
Read your HTTP referers. In the user's plugin list is a link to your site; you can even add additional links. Use an interesting link text to make people click on it.
<?php # -*- coding: utf-8 -*-
* Plugin Name: Add extra link to plugin list
* Description: Demo plugin showing how to filter the plugin links
* Plugin URI: http://wordpress.stackexchange.com/questions/94308/finding-wordpress-sites-using-my-plugins
* Version: 2013.04.03
* Author: Thomas Scholz
* Author URI: http://toscho.de
* Licence: MIT
* License URI: http://opensource.org/licenses/MIT
add_filter( 'plugin_row_meta', 'wpse_94308_extra_link', 10, 2 );
function wpse_94308_extra_link( $links, $file )
static $plugin_base = NULL;
if ( NULL === $plugin_base )
$plugin_base = plugin_basename( __FILE__ );
if ( $plugin_base !== $file )
// Run this only once
remove_filter( current_filter(), __FUNCTION__ );
$new_link = "<a href='http://wordpress.stackexchange.com/q/94308/'>See how I did this!</a>";
return array_merge( $links, array ( $new_link ) );
I have seen plugins adding a parameter to such a link like
'?ref=' . home_url(). This borders the privacy issues mentioned above.
Use an analytics software like Piwik to track incoming traffic. Your visitors privacy is more important than your curiosity. :)