Mugshot: Red Hat's open source social networking site
Social networking isn't exactly a sector I'd have expected Linux developer Red Hat to jump into (insert joke about geeks socializing here), but that's what they've done with Mugshot. Mugshot's stated purpose is "to create a live social experience around entertainment," and perhaps what's most notable about Mugshot is that it's open source, i.e. if you want you can download the source code to the Mugshot server and set up your own social networking site, and you can contribute code to the project as well. That's something you'll probably never see News Corp.,
Correction: Six Apart VP Anil Dash wrote in to remind us that LiveJournal, owned by Six Apart and one of the first social networking apps, is totally open source and has spawned many clones with millions of users, and that many of the web's most popular sites rely on its components (in particular memcached). Thanks, Anil!