A while ago I wrote a couple posts on using Windows Azure Access Control Services, however, that information has now become a bit dated. Recently Wictor Wilén has written a complete guide for using Azure Access Control Services (ACS) with SharePoint 2010. Check out this great series below.
Azure Access Control Services allows you to setup SharePoint Authentication to use one of several public authentication systems such as Windows Live ID, Google, Yahoo, and Facebook. This could be very useful to an organization that is looking to build an extranet or a public facing website that offers the user a personalized experience.