QualityStreetMap (qualitystreetmap.org) is a Quality Assurance tool for coordinating mapping work. It displays a large grid, now covering the whole world. Zoom to see it. Login with your normal OSM login (uses OAuth) and then click a grid square to indicate mapping progress.
Mailing list: http://groups.google.com/group/osmqa-discuss
V2 (current version)
|QualityStreetMap is not developed anymore since September 2014|
Current version is based on osmqa v2 (https://github.com/pgiraud/OSMQA)
There is no running instance, but the code is still available at http://code.google.com/p/osmqa/ under the terms of the GNU GPL license.
Early design inspiration was presumably taken from the Old osm-matrix tool which featured a similar assessment scoring of grid squares.