We asked this same question way back in August of 2011. Much of what was said in that previous post is still true and really worth the read (see this link). Over the years since that post the biggest change to the area has been the acquisition of the Royal Gorge lands by the Truckee Donner Land Trust which is evident when you compare the maps from 2011 with the current ones.
In this post we update the previous maps using the Placer County GIS system and also add maps of the summit area in Nevada County, thanks to the GIS system that Nevada County has implemented. The summit area still remains a hodgepodge of private and publicly accessible lands. The maps that we have prepared here are meant to used by anyone that wants to explore the Donner Summit area. There are literally thousands of acres of publicly accessible land on the summit and even much of the privately owned land is accessible to hikers and skiers. However, there are many acres that are posted by the owners and access is strongly discouraged. If you have any question about an area, take a look at these maps to see just what the status of the area you are interested is.
The maps below are for illustrations in this online post and we would recommend that you download the higher resolution maps in this pdf file to get a more precise picture of the parcel ownership of the Summit area. Both the plain parcel map and an enhanced maps showing private and public parcels are included for both counties.