This year, the FOSS4G (Free and Open Source Software for Geospatial) conference is in Portland, OR in September. They are expecting upwards of 1,000 attendees.
If you will be there, come to the Cesium talk: “What’s new in Cesium: the open-source alternative for 3D maps” (Friday, Session 2, Track 4, 2:30-2:55pm).
Patrick