The list (which uses "open source" rather broadly) includes: OpenStreetMap, OSGeo, TileMill, GeoExt, JQVMap, Modest Map, Stamen, MapNik, OpenGeo, and Cloudmade.
I can't speak to the accuracy of all of the content, but this is certainly misleading:
OSGeo is a collection of open source packages for creating maps and displaying them in browsers. Some of the projects are old and effectively obsolete, and some are newer versions that effectively replace the old ones.
OSGeo is described this way, per the website:
OSGeo was created to support the collaborative development of open source geospatial software, and promote its widespread use.