The Sonoma Land Trust is a non-governmental, non-profit land trust founded in 1976. Land Trusts exist to protect and preserve open lands through purchase acquisitions of land or conservation easements, donated lands, and stewardship. The SLT (as do many Land Trusts) works closely with governmental agencies, and will be able to provide you much information about open space in Sonoma.
The Sonoma Open Space District provides a map on their website which details lands conserved through the District and private lands which are in conservation plans. Here is an interactive map.
I have participated in education programs through the Land Trust Alliance (1600+ Land Trusts across the nation) and have met people associated with the Land Trusts in Sonoma. The next Land Trust Rally in this fall in Connecticut. Maybe I will meet some Sonoma foks again.