Low-Hop, a non-native species, has been added to the USWildflowers database (01/04/2011.)  Scientific name is Trifolium campestre.  It’s also known by the common names  Field Clover, Large Hop Clover, and Hop Trefoil.  Photo below was taken at Reed Branch Wet Meadow Preserve, Towns County, GA on April 30, 2010.

Low-Hop Clover, Field Clover, Large Hop Clover, Hop Trefoil – Trifolium campestre

Note: The photo I had originally posted here was of Medicago lupulina rather than of Trifolium campestre.  I replaced it with the above photo on 02/12/2013, after receiving notification of the error from Robert Flogaus-Faust.