Crowdvine is a tool that allows you to create your own social network in just a few minutes. It works, but *man* is it ugly. No I mean really, it looks like a thirteen year old cobbled it together while working his or her way through a "Learning Ruby on Rails" book.
I checked; it turns out the "thirteen year old" is an industry veteran, Tony Stubblebine, who you might recognize because he wrote the O'Reilly Regular Expression Pocket Reference that some of us can't live without. But the amount of press this fun prototype has gotten is disproportionate to its utility. It's spooky, actually.
Now, sites based on crowdvine and re-skinned by professionals are pleasantly functional and smooth. But clearly a lot of time has gone in to these custom versions. Nice framework, but rapid code deployment definitely doesn't translate to rapid site launch.