When I redesigned my website, I wanted to have it be more interactive. I wanted people to sign up for the website be able to comment on anything. I chose to integrate with Google Friend Connect (GFC) instead of making my own login system.
Take my website here for example. If you look toward the top of this site, you'll notice the "Sign in using Google Friend Connect" button under the BrianClifton.com logo. Click it and you'll be able to sign into using Google, Twitter, AIM, etc.
Once you sign in, you can interact more with the website. You can respond to this blog post with feedback, you can fill out a profile about yourself, and you can sign up for a newsletter. Google wrote all of the code for you which makes the integration easy.
Here are a couple of links if you're interested in reading more about GFC...
You can setup a Friend Connect community here:
http://www.google.com/friendconnect
I would also recommend checking out the Google "Social Web Blog" which has great information:
http://googlesocialweb.blogspot.com/
If you get stuck while integrating, there's a developer forum where you can reach out for help:
http://groups.google.com/group/google-friend-connect-developers/topics
My website is written in C# using ASP.NET and hosted on a
Windows shared account at GoDaddy.com, Inc. During the process of adding Friend Connect to the site, I ended up creating a .NET library that makes integrating with GFC simple. Essentially, this library has server controls you can drag and drop onto your web forms.
I've uploaded the source code for my library to a Subversion repo at code.google.com:
http://code.google.com/p/friendconnect-dotnet/