Basically if you look at their demo (best to look at the help / FAQ about
how to browse, it took me some time to figure out) then you'll see it stores
lots of numeric / statistical datasets. The browser lets you navigate the
datasets, then lets manipulate the datasets in different ways.
Its mainly aimed at social science communities, but I thought it was
interesting because of the browsing paradigm, and because the data is quite
heterogeneous. They use DDI metadata standard - see
http://www.icpsr.umich.edu/DDI/