CloudFlare has developed a plugin for Joomla. By using the CloudFlare Joomla Plugin, you receive correct IP Address information for visitors to your site. CloudFlare is a service that makes websites load faster and protects sites from online spammers and hackers. Any website with a root domain (i.e. http://www.example.com) can use CloudFlare. On average, it takes less than 5 minutes to sign up. You can learn more here: CloudFlare.com.
Note that if your web server is already running the mod_cloudflare extension, this plugin is not required.
UPDATED 2013/09/12: Converted integration to a Joomla Plugin (was previously a module), added support for Joomla 3.x, and updated with the latest CloudFlare IP ranges.