well known collaboratiave webmapping project is OpenStreetMap as @relet has expressed. beside this there are some mapping projects which maybe cant meet your needs. nevertheless i want to give them all, i have heard.
1.Google Map Maker maybe it is not completely opensource but it is collaborative mapping.
2.WikiMapia is a privately owned, online map and satellite imaging resource that combines Google Maps with a wiki system, allowing users to add information, in the form of a note, to any location on Earth. Users may currently use this information for free.
3.Map A List is a wizard for creating and managing customized Google maps of address lists.
4.WikiAssetMap is is a prototype to demonstrate how to build a collaborative map with complex content using free services. This could be used for an asset mapping project, a stakeholder analysis, a spatial survey or any other kind of repository that requires specific input (more data fields than a simple description) from many contributors.
5.WorldMap - Supporting Academic Collaboration with Open Source, Harvard’s Center for Geographic Analysis (CGA) is currently developing WorldMap, an open source web mapping system that extends the capabilities of their previous AfricaMap technology by using GeoNode as a collaborative spatial data infrastructure.
i hope it helps you...