Landing-like software from CUNY, based on Buddypress, intended to provide a learning commons with relatively little effort or configuration. It's a nice bit of packaging, slick, with good collaboration tools and a simple, activity-stream-oriented social network. Commons in a Box is definitely worth looking at if you need a site to support a bottom-up social community or network, and you don't have a wealth of resources to put into building your own. 

I came across this software because it is being used in the University of Brighton's newly reborn community site at https://community.brighton.ac.uk which, until it was killed off last year, used to run on Elgg.  I remain a fan of Elgg for building such things, which has a lot more options than BuddyPress available by default, richer access control, and a much more elegant technological design that makes customization more robust and flexible, but this seems to be a great simple solution that just works without demanding much effort, and that, thanks to its Wordpress foundations, could be customized to do pretty much anything you'd want a bit of social software to do.