A dense packing of circle dimers.  The particles are the union of two unit circles with their centers separated by 0.582.  This is the dimer with the densest known (translate or lattice) packing amongst this type of dimer <a rel="nofollow" class="external autonumber" href="http://www.springerlink.com/content/y3758v678781nwh6/fulltext.pdf">[1]</a>.  I believe this is the right structure, which should have a packing density of 0.936599...
<a href="//commons.wikimedia.org/wiki/User:99of9" title="User:99of9">Toby Hudson</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>