A New Editor for OpenStreetMap: iD
mapbox.com
mapbox.com
The Javascript libraries they're using are interesting. D3 is a big part of it, but equally notable is that there's no conventional slippy map library like OpenLayers or Leaflet. Instead they're doing something simple with d3.geo.tile, a relatively new component.
Particular attention is due to MapBox's OAuth implementation ohauth, purely client-side (no server proxy): http://mapbox.com/osmdev/2013/01/15/oauth-in-javascript/ They also use RTree (for spatial indexing), Lo-Dash (an Underscore alternative), and a few extra bits like JXON for XML, the async library Queue, and Bootstrap tooltips.
http://en.wikipedia.org/wiki/Wikipedia:VisualEditor
Not that this takes anything away from MapBox's achievement, and OSM were foresighted in choosing a sanely structured data model that allows things like this.
From the product page[http://ideditor.com/]:
"We’re building iD in pure Javascript, using d3.js and SVG for map display."
This has got to be the coolest non-graphing use of d3.js I have seen so far.
Anyone who hasn't should view the demo here: http://geowiki.com/iD/
I'm sure they'll add a lot of improvements eventually, but if you're looking for a more feature complete, more polished OSM editor in the interim, take a look at the ArcGIS OSM editor: http://www.esri.com/software/arcgis/extensions/openstreetmap