The Free and Open Source for Geospatial (FOSS4G) conferences refer to a series of events which provides presentations, workshops, and networking opportunities based around Free and Open Source Software for Geospatial.
Each year one annual global event rotates around the world. It is the premium Open Source Gesospatial Conference hosted by OSGeo, find the current and all previous conference sites here: http://foss4g.org
Each year a number of regional events which follow similar principles are held around the world hosted by Free and Open Source Geospatial enthusiasts. Read more about how to roll your own FOSS4G event here: http://wiki.osgeo.org/wiki/FOSS4G_Handbook