Although unsure, I believe this to be a member of the Xysticus genus of crab spiders. They are primarily ambush hunters, picking a likely spot for unwary prey to pass by. They don’t build capture webs but rather rely on their strength to overpower and hold insects quite a bit larger than they are. I’ve watch a female build a silken sac for her eggs, attach it to a milkweed leaf, and stay with it, protecting it.
This picture was taken September 3, 2011, in Green Lane, PA.