The benchmark for fixed rates is the 10 yr treasury bond. As the 10 yr bond goes down, fixed mortgage rates go down.
I use Freddie Mac's numbers:
http://www.freddiemac.com/dlink/html/PMMS/display/PMMSOutputYr.jsp
The 30 yr fixed peak in early July @ 6.79%.
Last Thursday, the average 30 yr fixed was 6.43%.
It's no coincidence the 10 yr treasury bond peaked around the first of July, as this graphic illustrates:
http://stockcharts.com/h-sc/ui?s=$TNX&p=D&b=5&g=0&id=p10401193626
The 10 yr dropped around 30-45 basis points, and the 30 yr fixed dropped 35 basis points.
On the adjustable rate side, rates are affected by short-term rate averages. LIBOR, COFI, and MTA are some of the short-term interest rate averages. Since June, rates have drifted lower for LIBOR, and CMT, but up for COFI and MTA. You can see these monthly numbers at
http://www.moneycafe.com/personal/allstates/interestrates.htm
So, it's not really a who but a what that sets our mortgage rates. As long as China, Japan, and dollar-rich OPEC nations buy our debt, our rates will remain at historically low levels.