I've noticed that for some locations, such as Tbilisi Georgia where I am now, that both Google Maps and OpenStreetMaps have incomplete data but with different omissions.
It would be really nice to be able to see them overlaid on one another so I can do things like zooming in on something in the Google satellite image to know I have the right spot on OSM.
I was sure I stumbled across a site that did just this once but now that I need it I can't seem to find it or anything like it via Googling.