Google is your friend.
Here is the Wiki article on the subject. The guys you need to read to understand Open Theism are Clark Pinnock, Gregory Boyd, and John Sanders.
One of the more colloquial terms for Open Theism is "free will theism," so the affinity of synergists for that theological construct is understandable. It is worth noting the leading Arminian apologist Roger Olson is not a supporter of Open Theism.
Here is an article he wrote explaining why he is not an open theist.