Friends, romans, countrymen... et al, the Setting Up a Sitecore Extranet article I wrote in 2011 is one of my most trafficked. It occurred that many of you could benefit from the additional changes that I've made since then, so that those who are setting up your own system by hand could now benefit from a module that manages most of the process for you. For visual reference, I've made a video for a quick intro to the module.
There already exists an extranet module on the Sitecore Marketplace but I'm not the developer of it. Sitecore is. The documentation says it was created in 2007 for Sitecore 5.3. I've never used it so I can't speak to its efficacy. I'm presenting my own solution and you're free to choose which will work best for you.
*(updated on 5/30/2014) For those interested, I have released an Extranet Module to manage creating and removing an extranet for your site.
I've recently needed to setup an extranet for several websites and began researching. There are several helpful articles on the topic. I found bolaky.net and blog.wojciech.org as well as using Sitecore's Security API and Security Administrator cookbooks. Although there was enough information to get me started, what I found was disjointed and there were a few specifics that weren't detailed. After some testing I was able to create a functioning extranet and in an effort to create a more comprehensive article I have detailed my results.