NLB Private Site
There are numerous reasons to have a private site, and regardless of if you want to make private your whole website or just a set of pages, NLB Private Site helps you to achieve this.
Your website's content will be secure from the prying eyes while they stay annonymous, but you may opt to allow search engine's spiders to crawl the hidden content.
This plugin also features an IP filter, which you may use to allow regular users to login to the site only from certain ranges of IP addresses. A special feature is IP filter bypass, which allows certain user groups to login from any IP address.
You may require a valid invitation code in order to allow visitor to access the registration page. Additionally this invitation codes can be disposable, so once they're used, nobody else can use such code to register.
These and some other features you can use for FREE in your Joomla website, as long as you use the core registration system.