Building a single website that serves multiple sites has been possible for a while, but it's always been a hassle to set up. Whether you were using D7's multisite features, Spaces and Persistent URL, Domain (Access), ... they all had their limitations.
No more! With Group Sites, you can set up your sites using the Group module and Group Sites will almost automatically take care of the rest for you. Oh and did I tell you about how fast it is yet? It takes the query access layer in Group and narrows it down to just the access checks for the currently active site. After all, why waste resources getting results you won't be needing anyway?
We initially built the module when a client of ours asked to set up a single site that would serve over 40 different markets. Because we thought it could serve so many more than just our client's needs, we made it more abstract and contributed it back to drupal.org. The end result is a module that can take a Group set-up and serve. each individual group and its content as a standalone site, recognized by either domain, path prefix or anything you can cook up yourself. You can even switch an existing multisite from one approach to the other without worry.
This session will include an explanation of how it works, some tips and tricks on setting it up and a brief demo showing you what it can do.